You don't get a limited number of fresh installs, as long as your licence is already installed, unless you are doing a complete uninstall including the registry settings. If you are doing a licence restore after every fresh install, you are doing it wrong.
It is most likely that you are installing a later version, which will work fine for the first run, and then later you are launching MC using the wrong icon, which refers back to version 48. That should only happen if you installed the new version in a non-standard location. Unless you change from the 32 bit to the 64 bit version of MC.
My guess is that you were running the 32 bit version of MC up to version 48, then you switched to the 64 bit version, which is installed in a different location to the 32 bit version. Then on subsequent launches of MC after the installation, you are using the icon for the old 32 bit version.
It is normal to be able to have both the 32 and 64 bit versions installed and runnable at the same time. You just need to sort out which icon in Windows to use to launch the new version of MC.
EDIT: BigSpider beat me. I'll leave the above though because it explains how it may happen.