If this is the only information you have than you have to make some assumptions. You have to estimate the highes traffic he is going to offer to the network, i.e. BH traffic.
We could say:
1,000,000 min in 30 days (remember that these days include weekends when traffic is usually lower).
1,000,000/25 = 40,000 min in 1 day (I divided by 25 and not 30 in order to compensate for weekends. The value is from my experience for Telephone networks).
40,000 * 12% = 4,800 min in Busy Hour (12% is also rather traditional value for telephony).
4,800 / 60 = 80 Hours = 80 Erlangs.
Here you have to use Erlang-B calculator. The input is 80 Erlangs and the GOS you want the customer to have, lets say 2% blocking. So we get 92 lines.
Good luck with making telecom business and have fun! 🙂