As far as I’m aware this is OK, though I’m only an avast user and not a representative of Alwil software.

Provided that license number is used on only one system then as far as I’m aware it is OK, if you have to Reinstall windows (or replace that system with a new one) just do it and use the same license key as before. Just ensure that you keep a copy of your keys securely and only use each key on only one system.