Does this mean that if you're on a remote stand, the bus will drop off its capacity then go and get more people then come back? I think that'd be a great feature for realism because having 250 people deplane from 1 bus all at once is a bit weird!
By default, the bus has a real-world capacity of 90 passengers. Some say the Neoplan bus can fit up to 156 pax, but we haven't modeled the absolute largest size variation but rather a standard one, and there are even different standards to evaluate the passengers capacity, IATA uses 4 pax per square meter, others use 5 and others 6, to arrive to a figure of 156 passengers so, we used a middle value of 90.
So, if the passengers to be loaded are more than 90, another bus will came to load the rest of them.
This, by default, but if you are busy, and don't want to lose the extra time required to wait for a 2nd or a 3rd bus, disabling the "Multiple Trips" option in the GSX Settings will make the passenger bus being "magic", taking an unlimited number of passengers, so there always be a single bus in this case.