There is no limit to the number of assistance dogs the bus driver can allow on a bus, as long as there is space.  An assistance dog must not block the aisle or travel on a seat.

The assistance dog if the responsibility of its owner and must be kept under control, whether it is in a harness, leash or carrier. 

You must make sure that your dog is wearing its harness or jacket so that the driver knows it is an assistance dog.