There are purchasers for the full lift capacity too, like ISS modules and major telescopes.
Geosynchronous satellites are an obvious case where satellites will collect into a limited number of orbits but they vary on what point of the Earth they sit over. Also getting to geostationary orbit takes a lot more fuel so the rocket has less room for payload than, say, low EArth orbit. I'm not sure one rocket can launch a geostationary satellite above the Americas and above Europe in the same mission.
But you can't really launch a satellite in a polar orbit and an equatorial orbit in the same mission, for example. Likewise, how economic is it to deploy one at 150km and another at 250km?
Starlink is a special case because it's a related constellation of satellites where a number of satellites are in the same orbit.
[1]: https://en.wikipedia.org/wiki/Falcon_Heavy [2]: https://en.wikipedia.org/wiki/SpaceX_Starship
It can. Geostationary satellites are a certain distance above the equator. If they adjust their orbit a tiny bit lower than that they start to drift east, if they adjust their orbit a tiny bit higher they start to drift west. This process is called "repositioning".
Generally there is a tradeoff between how much fuel you spend on it and how fast the repositioning is done. So you can do it quick and then your sat will have less fuel for position keeping. Or you do it "slow" and then you preserved more fuel potentially extending the lifetime of your satellite.
But these are all done with tiny bits of fuel (compared to the fuel needed to put the satellite up there in the first place) because the delta-v involved is very small.
Easily. Moving within an orbit is a matter of fine adjustment. For example, any stationkeeping that expands the orbit slightly will cause the satellite to "fall back" over time. Geostationary satellites are the best orbit for this, since every satellite in such an orbit essentially shares it with all others, differing only in position along the orbit.
Also if you're in a geostationary orbit to deliver one payload you have to leave that orbit to get to another geostationary orbit because there are other satellites in your way.