ORA-00018: maximum number of sessions exceeded

cause - Whatever value you specified for SESSIONS  in parameter file .sessions created till that value.

action - Increase the value of  SESSIONS in Parameter files.

Try This -

1. alter system set sessions = 'VALUE' scope=spfile;
2. alter system set sessions = 'VALUE' scope=pfile;
3. alter system set sessions = 'VALUE' scope=both;