I believe there is one bus with a capacity of four. It is usually not used, but may be available for a larger group if requested.
I know that we once rode on a bus with a larger capacity.In our situation, there were several buses in a row that had non-functioning lifts. After a while, there was a back up of people with wheelchairs and
ECVs waiting = I think 4 or 6. That's when they sent for the special bus.
I believe they still also dispatch it on request if there is a party with more than 2 wheelchairs.
This is from the disABILITIES FAQs information and was written for me by a bus driver:
If you are traveling with more than 2 wheelchairs, contact Guest Relations, or a Bus Supervisor, or ANY bus driver, and ask for one of the "Special Services Buses". There are two buses reserved for larger parties, with multiple wheelchairs. Each special service bus has FIVE wheelchair stations, and plenty of seating. These buses are especially helpful for groups visiting from schools for the disAbled. They can usually arrive within 20 minutes of being requested (although you are much better off making your request well in advance--a driver is not assigned to one of these buses until its needed), and will take you and your party wherever you need to go. There is no charge, and you may make arrangements, if needed, with your driver for future pickups.