The default value for NUMFORMAT is 10.
SQL*Plus will try to fit all the number data in 10 spaces of output if NUMFORMAT is 10.
SQL> create table t( my_column number );
Table created.
SQL>
SQL> insert into t values ( 1234567890 );
1 row created.
SQL> insert into t values ( 12345678901 );
1 row created.
SQL> insert into t values ( 12345678901234567890 );
1 row created.
SQL>
SQL> select * from t;
MY_COLUMN
----------
1234567890
1.2346E+10
1.2346E+19
3 rows selected.
SQL> set numformat 99999999999999999999.99999999999999;
SQL>
SQL> select * from t;
MY_COLUMN
------------------------------------
1234567890.00000000000000
12345678901.00000000000000
12345678901234567890.00000000000000
3 rows selected.
SQL> set numformat "";
SQL>
SQL> select * from t;
MY_COLUMN
----------
1234567890
1.2346E+10
1.2346E+19
3 rows selected.
SQL> drop table t;
Table dropped.