A 1. The number of TRX a BTS can handle depeneds in the BTS max config like 12,12,12 for a nortel S12000 or 18,18,18 for a s18000, but also dependes on your frequency plan.
A2. Trunk capacity are the E1s or T1s connected from the BTS to the BSC, could be another limitation for question 1.
A3. 2 Time slots per TRX are used in the trunk. consider a couple of sync time slots for the BTS. Nortel handles 14 TRX per E1.