There doesn't seem to be a limit on the number of times you can activate Windows using the OEM key. Earlier I read that multiple activations can get your key "flagged" but you can still opt for phone activation after that.
I have a laptop with Windows 8.1 OEM key embedded in the BIOS. I reinstall Windows every three months and haven't encountered a problem yet (been doing it for the past two years almost).
In fact, lately Microsoft has even added official support for reinstalling Windows using the Media Creation Tool. The tool which can be downloaded from here allows you to create an ISO file of the corresponding version/edition of Windows corresponding to your OEM key. The only advantage of drive image backups over reinstalls is the fact that you do not need to reinstall software every time. For the best experience, you should once do a clean install of Windows (since the default often comes with a lot of bloatware) and then make a hard drive image. The default configuration often does not even have partitions.