1st->U have to know the no. of total subscriber(let’s say 1000) and average erlang per subscriber(let,s say it 25 mErlang) in that area.
Multiplying these 2 values, u can get the total erlangs in that area.(1000*.025=25 Erlangs)
2nd->From the erlang B table, u need to find out how many channel u need to carry all that traffic.(Depending how much blocking u are allowed to give).Lets say ur allowed blocking is 2%,then required no of channel= 34)
3rd->From that no. of Channels u can plan the no. of TRX for that particular cell serving that area.
(In my example for 34 channel, 5 TRX can carry that traffic).
Hope,this will somewhat clear u.