the number of gprs users in one cell is theoretically very high.
what you can do is limit the number of TBF per PDCH (= number of gprs users per timeslot)
and also the number of PDCH per cell.
If I set 10 PDCH and 5 DL TBF / PDCH (case a)
And then later I go for 8 PDCH and 2 DL TBF / PDCH (case b)
that will limit the total amount of gprs users
(note: 1 user should get 3 or 4 PDCH per TBF)
but there is no “static” limit.
in case (a) you could expect about 15 users simultaneously, or more. (peak = 50)
in case (b, expect 3 to 5 users simultaneously. (peak = 16)
but that’s really not how you should start your gprs dimensioning. You should think in terms of throughput rather than the number of users.