Hello and welcome ^^.
I'm not sure of the capacity limit, but I can tell you, you dont HAVE to pre-register, but if you want to get the cheaper rate, then I would recommend it. Currently until July 31 (tomorrow O_O), the rate is $35. for all 3 days, and after that, it will go up to $40.
If you don't know the site, it's at http://ani-magic.org