This question seems to be asked a lot and it's really unfortunately that Microsoft isn't more explicit in answering it. You might be able to glean some information from this page or this page that allude to what might happen, but I would be lying if I said I knew for sure (I know, that's not much help).
There is a comment in the FAQ that answers the inverse of what you're asking, suggesting that purchasing subscriptions over the expected number of users won't be 'reimbursed' (ie. If you buy 10000 and only use 500) - so you might be able to extrapolate something from that.
My guess would be that the site access would be crippled, and/or you would be notified when you're reaching your threshold in order to be able to up your limit. If there's any uncertainty between 500 and 501 Anonymous or Authenticated Users accessing your site, it would probably be advised to estimate comfortably above this threshold, or to switch to a pay-as-you-go subscription model. But, because they will charge you for what you purchase, you obviously don't want to overshoot your estimate either.
I know that's not really a definitive answer to what you asked, but I hope it helps.
