If you don't have your own room, you will need to book the license with ITTS for your webinar. Not only will you need the license for the webinar, but for setup (so you can create the link to the meeting for your presenters and attendees, as well as for any practice sessions prior to the webinar.
The Registration limit in Adobe Connect cannot restrict users who have a direct link to the class using the "Anyone with the link can enter the room" permission setting in the meeting that event points to.
In order to limit the number of attendees, check out how to set the attendee limit and set meeting access to force registration.
Link to May 1st Session for Running Sessions for RUSA/ASCLA