bts capacity depends on how many TRX you put in it. (a trx is a transceiver board).
1 TRX will (in average) carry up to 7 calls (8 timeslots in 1 TRX, 1 for signalling + 7 for calls). In case of half rate, 1 TRX = 14 calls.
You can place up to 24 TRX in one BTS.
24*7 = number of full rate calls
24*14= number of half rate calls