We've stayed at POR quite a few times and yes, on especially busy days, the busses can be full and you may have to wait for the next bus but that was usually early in the morning towards park opening or right after park opening. I found it to be depended too on what park people were going to. Usually, busses for MK were the most packed but it's Disney and you have to expect that sometimes. At POR, we found by going to the bus stop at the front of the resort there was a much better chance of getting on the bus rather than by waiting at the stops towards the rear of the resort.