select count(*)
from t

select table_name, num_rows, last_analyzed
  from user_tables

SQL> SET TIMING ON
SQL> SET ECHO ON
SQL> SET LINESIZE 1024
SQL> 
Connected.
SQL> 
SQL> select num_rows, blocks from all_tables where table_name = 'USER_TABLE';

  NUM_ROWS     BLOCKS                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
---------- ----------                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
   6505460     125781                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           

Elapsed: 00:00:00.21
SQL> 
SQL> DEFINE n=1
SQL> 
SQL> select count(*)*100/&n as count_ from user_table sample block (&n);
old   1: select count(*)*100/&n as count_ from user_table sample block (&n)
new   1: select count(*)*100/1 as count_ from user_table sample block (1)

    COUNT_                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
----------                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
   7006700                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

Elapsed: 00:00:06.96
SQL> 
SQL> DEFINE n=5
SQL> 
SQL> /
old   1: select count(*)*100/&n as count_ from user_table sample block (&n)
new   1: select count(*)*100/5 as count_ from user_table sample block (5)

    COUNT_                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
----------                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
   6621140                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

Elapsed: 00:00:20.32
SQL> 
SQL> exit

select count(*) * 1e4 from t sample block(1e-2);

SELECT COUNT (DISTINCT PK_id) FROM TABLE