Would the panel supply stores do a rattle can colour match?
There are some, but I think most paint panel shops won't have the capabilities to put the paint into a pressurised can.

Also, is one can of colour coat enough for the guitar?
Theoretically yes, but it's not necessarily that straight forward.
Firstly, cans come in different volumes depending on brand (eg: 150g, 311g, 340g, 400g). It also depends on if you're doing just a gloss base coat or base coat plus clear coat.
If doing a single gloss base coat only, I would be concerned about potential for sand-through even with a single 340g can. I have most always done base + clear and I always go with extra clear just to be sure. When I was still using rattle cans as a primary method, I always used 2 - 150g cans at least for the base coat. Results vary between basic solid colours and metallics also.

With aerosol paints, they work out to be (roughly) 33% paint (1/3) 16-25% solvent, and the remainder is propellant (basically compressed air) so, not a whole lot of paint in reality.