If I have 10 Odoo users, but only 2 will call from Odoo, 1 will use Odoo Phone, and 1 users a hardware phone, how do I pay?

Payment is made according to the number of users who use both the PBX and Odoo, so called PBX users. These users can click to dial and calls are also assitiated with them.

So if there are 10 users in Oda, but only 2 entries in PBX -> Users section, only 2 users are charged.

Further. If one of the PBX users has a WebRTC SIP channel, then he also becomes a Phone user, and the payment for Phone user is charged.

So, in this example, the charge will be for 2 PBX Users and 1 Phone User.