You said rough estimate right???
O.K. Even a third world phone comapany, on a real tight budget will not go bellow 10 %. That is a farming comunity, where phones are used more for the emergency services than anything else.
That means that out of a hundred users, only 10 could be talking at the same time.
It is almost a perfect recipee to get you shot when you show up on that community on your phone company truck.
In small towns, out on the sticks noone on their right mind will go bellow 20-25 %. And that was before dial up internet.
All the way to 100 percent on a big metropolitan area, like downtown or financial district in a capital city.
You could not afford to have banks blocked by teenagers chating to each other.
As you could see, there is no streight answer to your question.
You need to know how much they talk to know how many lines to install.