Upper limit on simultaneous streaming from a single AppleTV+ subscription?
According to the Apple Support document Download and stream shows and movies from Apple TV+ and Apple TV channels, 6 streams at the same time are the limit for the normal subscription:


  Apple TV+: One subscription allows you to enjoy 6 simultaneous streams.


Since they say, that 6 streams are allowed, multiple streams per Account should be possible. Otherwise they would state, that up to 6 Accounts could be linked to the subscription.

But restrictions apply to students with the cheaper subscription:


  If you have an Apple Music student subscription, which includes Apple TV+ for a limited time, you can only stream Apple TV+ on one device at a time.