I have patched one of my wall from a few holes(5) where each is about a finger big. Since it is not a big surface area, can I use a spray stain sealer and primer, and then apply paint over the entire wall as usual? I'm not sure if it's worth getting a 1L primer that I will probably not use again.
Yes. The purpose of primer is to seal the raw plaster/gypsum/drywall and create a better surface for finish paint that won't soak it up.
As long as the rest of the wall is properly cleaned before painting (paint doesn't bond well to dirt, grease, etc), your finish coat should look even and consistent.
For your situation, assuming the existing paint was rolled on, I would spray on the primer and then go over the freshly sprayed area with a roller - going heavy on the primer and using the roller to spread it out nice and even with the same kind of roller texture the rest of the wall has.
Follow with one, maybe two coats of finish paint and you'll never know the wall was patched.