If you have a valid FusionReactor cloud or on-prem subscription for FusionReactor then you are always entitled to run the latest version of FusionReactor.

If you have a perpetual license (you own the license forever, but must renew your maintenance to get newer versions of FusionReactor), then the maximum version you can run is dictated by your maintenance expiry date.

You can find out with version of FusionReactor you are entitled to on the upgrade page.