It is vendor specific, Motorola has combined 64K (or more) known as RSL for every site and Nokia has this signalling distributed for every TRX and finally OMU.
With motorola you can go upto 14 TRX (0-SYNC, 1-RSL, 16-O&M, 29 remaining so you can connect 14 TRXs max).
What equipment do you work on ?? and what technology 2G or 3G.