Dan,
I think Heather had a thread on this. You can see this guide for help
http://hooshmarketing.com.au/blog/event-marketing/four-tips-to-building-a-limited-capacity-event-in-marketo
the reality is there is a manual component if you fully use Marketo for the event. If you are also using cvent or eventbrite, you can have those systems manage that.
My recommendation would be to
- add a Wait Listed status to your events (similar to the one on Webinars)
- add a Reg # as a custom field
- follow directions in the article above.