I have not looked at sharepoint meeting room templates, but if it lets you limit the number of acceptances, it will work. There obviously is some way of doing it on the room side (not in Outlook) where rooms can be full. I don't recall this ability on the resource cmdlets for Exchange 2010 - but doing it in the room, rather than in the organizer's client, would be the best option.
I have code that will count the # of invitees and let you set a limit on the invite, but that is fairly basic code. Counting acceptances (in the organizers client) and not allowing more will be more difficult and not for the beginner programmer.