TL:DR; Within five years you will almost certainly be using the Flex pricing model (explained below).
The Flex pricing model charges you per page generated. There is no restriction or charge on the number of servers, cores per server, users, functionality, or anything else. It's just per page. So generating 1,000 pages on a single server is the same as generating a 1 page report on 1,000 servers. Flex has a monthly charge that includes your first N pages and then a per page charge over that amount. There are multiple plans based on the included monthly pages.
The PRO pricing model charges you per server. On a server using a PRO subscription you can generate as many documents & pages as you wish (and the machine is capable of) for that fixed monthly fee. But you are limited to running it on one server per PRO subscription. A second server requires a second PRO license. The PRO license does include allowing running on a 2nd computer on a per-page basis (PRO is really PRO+Flex) but those 2nd computer charges can add up fast. The 2nd computer charge is designed for the case of bringing up a 2nd server and then 5 minutes later turning off the first server.
PRO is built around the past, where an application ran on a server, you purchased a license for the server, and that was everything.
Flex is built around the future where you don’t know how many instances of your application is running, or how long each instance runs. And this abstraction will increase even more in the future.
And that is why we recommend Flex for your licensing. It’s useful now for any occasional use on additional systems. And it’s necessary for your future deployments.