By default you get access to 500/requests an hour. Active subscriptions unlock 5,000 requests for $35/every 3 months.
If I force people to Login with Xbox and later decide to disable OpenXBL how will they login?
If you later decide to disable OpenXBL, it's okay! New users who sign in with Xbox still have a local account (they just do not have a password). All they need to do is click "Forgot/Reset Password," type their email in and XenForo will handle the rest.
If I force people to Login with Xbox and they don't have an Xbox account - what happens?
No problem! They can create a Microsoft account for free if they do not have one during the Login/Signup process. If they do not have an Xbox account they will land on a splash page. This page will give detailed instructions to go to www.xbox.com and login with Microsoft account they just made. They'll be asked to create a free gamer tag. After that, click "continue" on the splash page and they will get logged in!