The actual number of DIDs that can be pointed to a T1 isn't as important as the traffic calculation for your users. You can have an almost unlimited number of DIDs pointed to a T1, with the hard limitation of the number of channels for concurrent calls. If the people in your environment are heavy users, such as call centers, the ratio of trunks to users is about 2 to one to 3 to one. A more modest environment is 4 or 6 to one. 7 or 8 to one, is an environment with lighter use.