If I set subscription level at 5,000 active users what happens when we hit 5,001? Is that user unable to login or does the subscription level automatically increase?
To my knowledge there would be no automatic increase in the subscription level and this would also not be an hard limit. This would meant that you would have time to increase the subscription yourself without disrupting user authentication.
For complete details, given this is not a technical question, you may be better served by contacting sales; you can do so by following the respective option in the pricing page.