Multiple starports tho are canon. An example of Rhylanor from the MT era showed two orbital A-class, and two down side, with multiple B and C-class for freight and such.
Population is a factor, as well as market. IMTU, multiple starports downside are expected. IMTU, on worlds of pop 3 or less have a single facility, and not always then--there are always exceptions.
Generally, the "extra" ports planetside are one class less than the main facility IMTU.
Planet's tech level administrates mode of transportation of goods distributed landside. So guess what, that A-class port on a TL-3 world? Possible--but don't expect goods to arive by truck--IMTU, probably animal transport and wagons. Loading of containers at port, manpower intensive and few forklifts.
Hey--I just loaded three wet ships with an army brigade's worth of containers and 2000 vehicles and trailers last month.. when the port's vehicles are strapped, good ole manpower is the next tool of choice, and this is at TL-8+ in RL.