for Abis transmission ? you have to define how many Abis timeslots are used per BTS. Usually:
one cell with 4 TRX = 9 Abis ts
one BTS with 3 cells = 9×3 Abis ts + 1 Abis ts = 28 Abis ts
And you have to add one Abis ts per Abis link
So in the end a BTS 3×4 = 29 Abis timeslots for voice only.
For reference, one full Abis link = 32 timeslots.
Then you must top up the Abis timeslots reserved for GPRS, which is a very complicate matter.