If you RSVP to an event, and it is confirmed, can they change the amount of people to attend after confirmation

Probably yes, the hosting organization can limit who attends the event. Especially if there is a limited amount of space or supplies available, the event is not suitable for certain people (like children), or the hosting organization is picking up the tab and can only afford a certain number of people. Mistakes do happen.

You don't say what kind of an event. An invitation is usually just an invitation and does not grant you a license to be at a place, as a ticket often does. Even with a paid ticket, usually the license can be revoked at will by the venue, with or without refunding your ticket price, depending on the terms of the ticket and the situation. So if you have merely been invited to attend something, RSVP, it was confirmed, and the next day were told you were limited to two guests, that seems reasonable and legal, even though you were disappointed. In life, not everything that is disappointing or hurtful is illegal. It sounds as if the people running the event wanted to let you know as soon as possible so you and your family would not be as disappointed.