1000 people.
70% market penetration = 700 subscribers
25 mErl per person (at busy hour)
700 * 25 = 17.5 Erlangs for this “1000 ppl” area.
17.5 erlang can be handled by 4 TRX. (4 TRX can handle up to 21 erlangs)
That’s just approximation : you have to know your penetration rate and the BH subscriber average traffic.