At https://www.3cx.com/ordering/pricing/ for example they have you input the number of users and it guesses at the number of simultaneous calls. That's basically a guess/average...one can have 100 users that rarely make calls, or 5 users with 30 calls queued up on a busy day.
I was planning to delete this thread but it was too late. That link with # of users often confuses me/ Oy should just change it to # of simultaneous calls and het rid of # of users