The BTS capacity depend on its configuration i.e. TRXs installed.
As you know, GSM system is a TDMA and thus each TRX can serve 8 subscribers @ once (eahc one will occupy a TS)
Thats means if you have a 333 BTS then you can serve 3*8 + 3*8 + 3*8 subscribers at the same time.
If you mean to say how many subcribers from Erlang-B point of view then it is the traffic offered divided by the average mErlang per subscribers…