There's no hard limit, you'll be limited more by memory and CPU load on the router. If you have a VPN module in the router, then your CPU woin't be getting hit, so memory will be your main constraint.
Can't give you a reliable figure though, cause it'll depend on how much traffic each user is sending, what encryption they're using, what packet size they're sending, etc, etc. Your best bet is to monitor the router to see how it's performing as your users connect in. Personally I wouldn't go with more than say, 20 users on this size router, but as I said, it really depends on how much traffic and what type you're pushing through it.
