Is there a way to limit registration so that we don't go over our license?
You can look at a 3rd party events tool (we have used ConnectYourEvents (CYE) and ConferenceEdge (CE)), or wait for Connect 9, as the events module in it has registration limits. Feel free to PM me if you have questions about CYE or CE.
The only thing is that we use the Eloqua Connector to send out the emails and Eloqua does not integrate with Events - only rooms.
As far as I know there is also no way to query the number of registered users through the Eloqua connector. So we may be stuck for the moment.