Hi,

you can put constraints on the res.users model as like below.

_constraints = [  (_check_no, 'You cannot create more then 2 users .... ! ! ! !', []) ]

def _check_no(self, cr, uid, ids, context={}):

    user_ids = self.search(cr, uid, [] )

    if len(user_ids) > 2:

        return False

    return True

I hope it will help to achieve your goal.