Just remember to assign the license to your Webinar user, after you purchase it.
Also remember to check for any registration-limit on your Webinar. This is a separate number. You can set it to anything you want, but I have seen it default to the (maybe original) capacity of the Meeting/Event.
I have a similar question. Where do I find confirmation that the capacity increased to 500? My general account settings say 100 and I could not find confirmation in the Meeting settings. Thanks, where do I find confirmation????