I know there's no way to cap registrations in Marketo. So far from what I've been reading is that I can set up an email alert to send out to me once the max number of registrants fill out my registration form.
I'm trying to follow the directions here
Four Tips to Building a Limited Capacity Event in Marketo » Hoosh Marketing
But I'm trying to setup Send Alert step in a Smart Campaign and I can't choose my registration form or registration list in choice 1. I only can choose the lead fields.
In my Choice 1 I want it to do this: If "Event Registration Smart Campaign or List" is "120" Email "Event Registration Full Email" to Me
I think you could make a 'scoring' field and have it add up the number of registrants in your registration trigger. Then, when the field reaches "x" number, you could have the alert be triggered to send. The only limitation I could think for this would be that you'd have to reset it per event and/or only be able to use it for one event at a time.
I also know there's testing for Smart List subscriptions occurring; you could ask to be part of that and simply subscribe to smart lists.
Kristen has the right idea. Generally you cannot do this in Marketo.
If you do a search, there are several interesting solutions. Given the time to make them work, I recommend just watching your program or setting everyone to Wait Listed until you Register them.