The Get # of Attendees needs to be selected and Max # of Attendees entered BEFORE the Event Registration is fully created otherwise it will not retroactively count already registered attendees within the count. For example if you have a Max # of Attendees at 50 but 25 have already registered.
You now enable the Max #, you should only enter 25 for your Max # because there are already 25 out of 50 already registered and you do not want more than 50 total registering for the Event.