I do not understand how much will I be charged if I upgrade my account.

So far I'm using free princing for testing but my aim is to create different accounts on different domains as the example you'll find below:

I need to manage abc.com and dfg.com domains with 2 email accounts each.

I dont understand if I will be charged for 4 users or only 1.

Could you please clarify my doubts in order to upgrade my account?

Thanks in advance.