How to increase no. of sessions in oracle database?

If you are increasing sessions parameter you should consider increasing processes and transactions parameter as well.

Here is the formula you can use to determine their values.

processes=x
sessions=x*1.1+5
transactions=sessions*1.1

E.g.

processes=500
sessions=555
transactions=610

Commands:-
sqlplus "/as sysdba"
or
sqlplus '/as sysdba'


show parameter sessions;
show parameter processes;
show parameter transactions;
alter system set processes=500 SCOPE=SPFILE; 
alter system set sessions=555 SCOPE=SPFILE; 
alter system set transactions=610 SCOPE=SPFILE; 

and restart the database.

Settings for higher performance:-

alter system set open_cursors=1500 scope=both;
alter system set processes=3000 scope=spfile;
alter system set undo_retention=10800 scope=both;
alter system set max_shared_servers=50 scope=both;
alter system set db_files=2000 scope=spfile; 
alter system set sessions=3305 SCOPE=SPFILE;  
alter system set transactions=3635 SCOPE=SPFILE; 

If you facing below issue while doing alter system
SQL> ALTER SYSTEM SET memory_target = 0 SCOPE=SPFILE;
ALTER SYSTEM SET memory_target = 0 SCOPE=SPFILE
*
ERROR at line 1:
ORA-32001: write to SPFILE requested but no SPFILE is in use

then perform 'CREATE SPFILE FROM PFILE' to create a SPFILE and do restart the database.