Uh............well, things like trucks tend to need larger heavier clutches. This requires a larger diameter flywheel, and there's no way around it----larger flywheel == more teeth.
On Chryslers, the starter mounts to the bellhousing, so the bellhousing must match the flywheel you use, and the starter mount compensates for this
Just to add to what the previous poster stated,if you have a larger or smaller diameter flywheel then you need to add or subtract teeth to keep the same tooth spacing on the flywheel in order for the teeth on the starter to mesh with the different diameter flywheels.