So my MacOS just ran an update and it required me to enter my Key (it’s done this before). When I did it told me I have no more activations available. Having I been chewing up activations on OS updates and not known it?
How do I go about fixing this? It’s the same machine that had a previous installation as of this morning.
Submit a ticket to GP support.
Also, see this article for background
Excellent, thank you!
You should be able now to activate normally 
This explains it. I have had to re-enter my license key on several occasions. It’s not a big problem but I wondered why.
I have been advised not to update the Mac OS before deactivating the GP licence and re-activate after the OS update. So I have now removed automatic updates to avoid this issue.
Yes - also make sure that when you do perform an update, don’t do it unless you have plenty of time to deal with any other issues you encounter after an update. As mentioned elsewhere, last thing one needs is to discover that a plugin that used to work fine suddenly doesn’t work any more after an update and there’s no time to deal with it before going up on stage.