What is the maximum number of Apple devices you can have for a single account?

That depends on what you mean. You can have ten devices authorized for purchases in iTunes/App Store:

View and remove your devices that are associated with Apple ID purchases - Apple Support

But that's not a limit of devices you can have signed into your Apple ID. I don't know that Apple has ever said there is one for that. Apple has said that Apple IDs should not be shared.

At the moment I have two MacBook Pro's, Two iPhones, 2 iPads and a Apple TV.

That being said if I recall a few years ago they only allowed 5 devices per account.