How to view, how much clients can connect to 2911/k9 by vpdn(pptp) at same time?
Which ios is need to enable mppe on 2911/k9?
There does not seem to be any "hardcoded" limit to the maximum number of clients in IOS. However, several things may limit you:
Regarding the IOS, 2911 will use universal images, so MPPE should be included. You can verify with the feature navigator.
Pay attention to the licensing as it seems that this will be your limitations.