While there is a difference between "connecting" a number of VPNs and actually transferring traffic at a certain rate sustained/burst, encryption level, and other factors, the 425a can easily handle 100 VPN tunnels without breaking a sweat. You would have to add far more before you would likely run into a resource problem. There is no "fixed" limit for allowed VPN tunnels, however if you are already near the performance limit with web/mail/ and hundreds of simultaneous users, that could be the only deciding factor.
