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'


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.