I saw pictures earlier this fall, and unless they have added some new accessible Minnie Vans recently with s different configuration, the accessible Minnie Vans only take a total of 4 guests (plus driver = 5). It was 1 passenger in the front, 2 in the liddle row, plus the guest in a wheelchair (ECV). WDW did state they would send another vehicle for parties up to 6 guests, since that is what the non-accessible vehicle can take. I have not read any reports of that situation, and what I gather is the regular Minnie Vans are quite popular to the extent that some guests either have to wait or do an Uber/Lyft instead. So for a larger party (5-6) I would caution how easily/quickly they can provide that extra vehicle.
Enjoy your vacation!