the number of calls is not really relevant, what matters more is the volume of traffic (erlangs)
25mErl / subscriber @ BH x 2 millions = 50 000 erlangs
assuming average call duration of 50 seconds (rather realistic )
50.000 erl / 50 = 1.000 calls
So 2 millions subsribers (typical subscribers) will generate about 1000 calls of 50 seconds, at busy hour, during one hour.
To handle 50k Erlangs, you need about 600 BTS with 4/4/4 configuration 🙂