is the a limit on the number of devices authorised for an apple ID

It is indeed 10 -> See and remove your devices that are associated with Apple ID purchases - Apple Support

”More about associated devices

You can have ten devices (no more than five computers) associated with your Apple ID for purchases at one time.”

Hi notheapp,

I believe it is 10 devices at most associated with an individual Apple ID, from the perspective of purchases, etc. I remember that from Mac OS Catalina days and though I cannot find the supporting documentation on Apple's site, still believe that to be the case. If you are asking due to family needs, that is where Family Sharing can be handy.

Links of interest:

https://www.apple.com/family-sharing/

https://support.apple.com/en-us/HT204976

Cheers,

Anthony