This is always a bit confusing for people, so you're not alone. The short answer is "probably not". Here's how phone billing works:
There is a flat monthly charge for XX simultaneous calls - this number is typically either 4, 8, 16, or 32. This includes all of your phone calling, the server hardware maintenance and licensing, and Pinnacle's service to manage the system.
Note that because of the nature of the system, there is no billing distinction between "internal" and "external" calls. So, additional extensions will not cost you anything additional per month, but in so much as it facilitates additional phone traffic, there might end up being an indirect cost associated with more extensions if you start hitting the limit of simultaneous calls.
Also note that each "call" counts only once - if two "internal" extensions are talking to one another, that only counts as a single call for licensing purposes.