It appears it is one Mew per Pokeball Plus, I received the Mew on Lets Go Pikachu, then started a game on Lets Go Eevee, delivered Oak's parcel to unlock the functionality to communicate with the Pokeball Plus, however was told "This Pokeball Plus doesn't contain any Pokemon presents."
I then went back to Lets Go Pikachu on and put Mew on the Pokeball Plus to go for a walk, then tried again to receive him on Lets Go Eevee, however was told the Pokeball Plus doesn't contain any presents.
You also can't return a Pokemon from a walk on a different game, otherwise you get told someone else Pokemon is on the ball and the results from the walk will be deleted.
Screenshots below of when i received my Mew:
This appears when the Pokeball Plus is connected for the first time:
The receiving Mew process:
Message that is displayed when trying to obtain a Mew on a different game, after already receiving a Mew:
Message displayed when trying to take a Pokemon from a different game for a stroll with a Pokemon already on the Pokeball Plus from a different game:
If nothing else screenshots to preserve as much as this unique event as possible. It's going to be a rare thing getting an event Pokemon on a unique piece of hardware like this.