Search notes:

Oracle: Init parameter SESSIONS

The value of sessions is derived from processes.
According to the documentation, the formula is processes*1.5 + 22, however, on «my» databases, I found the formula is processes*1.5 + 24: 
select to_number(value) * 1.5 + 22 from v$parameter where name = 'processes' union all
select to_number(value)            from v$parameter where name = 'sessions';

See also

select * from v$resource_limit where resource_name = 'sessions';
init parameters
session

Index