Same question asked by Ahmad safi

I answered him following 

You create a model having 2 fields user_id(Many2one, unique) and user_creation_limit(integer).

That may have tree view and form view.

Then override the create method of res.users

@api.model
def create(self, vals):
    # Calculate this user is eligible to create
    # using self._uid    
    # by fetching list of all users and their created user id(create_uid )
    if eligible:
        return super(ModelName, self).create(vals) 
    else:
        raise UserError(_('Your limit is exceed'))