A lot of customers upgrade to Pro thinking every FB page attached to their account will not be on a Pro plan.
ManyChat sets up one subscription per Facebook page. This is to help our agency customers have separate billing for each Facebook page they manage. As such, when you upgrade to a Pro plan, you're upgrading ONLY the ManyChat subscription for that particular Facebook page.
Try to think of your ManyChat Dashboard as a hub for all your ManyChat accounts. You can manage an unlimited amount of Facebook pages with ManyChat.
The Dashboard is where you manage all the individual accounts; just keep in mind each new account (i.e. connection to a Facebook page) has its own billing associated with it.
Hope that helps explain billing a little better!