How to increase no. of sessions in oracle database?

If you want to increase 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'


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

and Restart the database.

Below parameters values also must be changed.....

sql>alter system set open_cursors=1500 scope=both;
sql>alter system set processes=3000 scope=spfile;
sql>alter system set job_queue_processes=4 scope=both;
sql>alter system set undo_retention=10800 scope=both;
sql>alter system set max_shared_servers=50 scope=both;
sql>alter system set db_files=2000 scope=spfile; 
sql>alter system set sessions=3305 SCOPE=SPFILE;  
sql>alter system set transactions=3635 SCOPE=SPFILE;