Usually buses have the same number of windows on both sides. The obvious exceptions are those with wheelchair lifts, or side emergency doors (often on both sides but occasionally just on one side). More rare are the ones with secondary entrance doors. I saw one from somewhere in NY that had an entrance door on the left side, just behind the driver (something about being specially bought and equipped for a route with stops on the left side of the streets on one-way roads).
You're right, though. Usually if buying a near-full length bus, districts will go on and buy the regular full length bus, to allow for any future growth on that route.