If you want to increase “sessions” parameter you should also need to increase “processes and “transactions” parameters.
A basic formula for determining these parameter values is as follows: processes=x sessions=x*1.1+5 transactions=sessions*1.1
1. Login as sysdba
sqlplus / as sysdba
2. Check Current Setting of Parameters
sql> show parameter sessions
sql> show parameter processes
sql> show parameter transactions
3. These paramters can't be modified in memory. You have to modify the spfile only
(scope=spfile) and bounce the instance.
sql> alter system set processes=500 scope=spfile;
sql> alter system set sessions=555 scope=spfile;
sql> alter system set transactions=610 scope=spfile;
sql> shutdown abort
sql> startup