One TrX can provide 8 Channels over Air interface depending on your project some channels are used for signalling and the rest are allocated to TCH. (for signaling 16,32 and 64kbps rate are available)
12 TX as far as I know provide 12*2=24 TCH if 32kbps signaling used then + 1 omu =31 ch
so we have 24 Traffic channels so just put in ErlangB formula with GOS=.01 (For core GOS=.001)
the important matter is how many TCH you have —> use Erlang formula
Hope it comes in use!