I ususally just plug in a Ratio.
A 4 to 1 (users to lines) is a good place to start.
I have one client with 40 or so extensions that had 12 lines, tried to expand to 16, but the vendor messed up and knocked them down to 8 lines only. Thus a 5 to 1 ratio, and they are doing just fine. (Once in a while I see an "ALL LINES BUSY" condition, but not ofter)
So, like I said, 4 to 1 usually, but depending on usage, you may want to go lower or higher.
What is the application?
Another monkey wrench in the plans is the CELLULAR PHONE.
A lot of personal calls now don't clog up the office phone system and thus, the ratio is squewed once more....