Hey Avinash,
1PCM–>2048Kbps(32 Channels)
1–Framer,1 Signalling.
So remaining 30 channels –30*64k=1920Kbps
Abis Signalling needs 64k;
Speech (4x16k TCH are multiplexed in 1x64k): means in 1xTRX—->128k
So 12 TRX—>12*128= 1536K
So 1PCM is more than Enough for your 12 TRX config