Modem Sharing

We are running a 70 user network using windows NT server on a true server. We are also using Microsofts Modem Sharing software to share 4 modems (comtrol 4 port Rocketport II). We have recently added another 4 port Rocketport II modem and are having problems adding the additional modems, and have some a question:

How many modems can you add to the pool?