It depends on what they install for you
ONT 612's are popular, but if you're needs are larger, then take a look at the link and it will tell you how many.
looks like 2-4 on average
The parameter you're after is called "REN". As the previous poster points out, the "612" model seems popular and it has an output rating of 5.0 REN. You can find the REN rating on the base of each phone -- add them together and as long as you're below the number indicated for your ONT, you're good.
A better way to solve this is to use a wireless phone system with a single base station. In this scenario, only the base station itself counts toward the REN total. Each individual wireless handset adds no additional line load.