Why can't I set oracle sessions value to 35?
SESSIONS


  Default value Derived: (1.5 * PROCESSES) + 22
  
  ...
  
  Oracle uses the default value of this parameter as its minimum. Values
  between 1 and the default do not trigger errors, but Oracle ignores
  them and uses the default instead.


If you want to decrease sessions, you need to decrease processes as well. Still, if you decrease processes, you will have plenty of 'free' sessions, and you will need to open several sessions until you receive the error.

Easiest way to simulate the error without messing with processes and opening several connections:

alter system set dispatchers = '(PROTOCOL=TCP)';
alter system set shared_servers = 1;
alter system set shared_server_sessions = 0;

$ sqlplus user/password@\'127.0.0.1:1521/orcl:shared\'

SQL*Plus: Release 12.1.0.2.0 Production on Sat May 21 11:38:41 2016

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

ERROR:
ORA-00018: maximum number of sessions exceeded