What users u mean? U mean.. internet users and internet BW? if yes, To my knowledge there is no formula which gives the number of users for a particular BW. It’s basically sharing.. of the bandwidth to the avaliable no. of users.
If it’s a trunk, I mean for voice calls… it’s always 64kbps per user with Toll quality voice. But now, most operators compress the voice, since we’ve better compression technologies, with same voice quality. So, depending on the compression technique used the no. of user varies for a T1/E1 line or for a BW.