Alas the answer is “it depends”.
Each paid plan comes with an included amount of usage, in USD. For example the $29/m Launch plan comes with, yep, $29 of usage.
What that $X of usage equates to is up to you. Each size of vm, each amount of RAM, each GB of storage allocated, and so on. You can check the pricing of resources on this page: Fly App Pricing · Fly Docs
For example if your allowance is $29, you could have ~14 of the smallest vms (costing ~$1.94 a month). Or fewer of the bigger vms if you want more cpu and/or more RAM. And of course you are free to exceed the included allowance and pay accordingly 