Yes - we do offer enterprise level subscription plans with more than 50,000 sign-ups. Please see the quarterly plan below (we also offer monthly plans).
Check out more details on our pricing page. Please click on "View bigger plans" link at the bottom of Growth (15,000) plan.
If you have a specific requirement or any specific number of sign-ups that you are looking to achieve, we'll be happy to discuss it with you. Please schedule a free demo session with us using our calendar, and we can discuss the specifics.