There is no public information related to the number of VMs per PPG. Taking into account that under an Availability Set you can have up to 200 VMs and PPG requires all VMs to be part of the same PPG (the limit of the max number of VMs per availability set), you end up with a limit of 200 VMs per PPG.
I would recommend opening a ticket from the Azure Portal and requesting additional information. When the limit is not publicly available, I recommend asking the Microsoft support team.