Two, maybe 3? I think they have two slots with tie downs and a third where you back up against a padded back rest to hold you in place when the bus stops. But since that's for wheelchairs only I'll go with two.
I really did hope they would increase the number of ECV/wheelchair spots but it looks like all they did was add the extra spot for a non tied down wheelchair user. I've used that extra spot and it was fine for me but I don't need much to feel secure.
Here is a video of the bus interior. At 9 seconds in you see the pad for a wheelchair user to back up to. Then it pans around and you can see the 2 tie down spots.
Year round mostly just pop and aoa when busier they have sent one to one of the port orleans resorts.
They can't fit in all the resorts like all of the Epcot resorts and magic kingdom ones. They just use it for magic kingdom and sometimes animal kingdom well at least they did some work to take them. So sure once pandora opens they will use them their more.
Dhs and Epcot still need to be reconfigured and worked on to take them. When it slower they my not use them but most days their at least two running if not more. They replace two buses on the route so it saves Disney labor and money and frees up buses to handle other routes or back up other routes. So they will use them year round.
It a small fleet of 20 so it plays into the two maybe three resorts when busy. Disney could technically do both pop and aoa same time during slower times with atleast four running.