Does anybody know if I want multiple licenses of Cubase 13, can I still use the upgrade from 12 multiple times?
Can you clarify what you mean exactly?
You can upgrade every license only once. The upgrade replaces the previous license with the new one, i.e. the Cubase 12 license is gone and it can’t be upgraded to 13 again. But you can of course still use Cubase 12 with the Cubase 13 license.
If you own multiple licenses of Cubase 12, it’s possible to upgrade each of them separately.