Look at the back of the sandpaper packaging and there's probably a rough guide. I use 320 grit to sand the primer. Thing is, you want to fill imperfections, create a base for the finish to adhere to and get it as flat as possible.
When you're satisfied with your final primer coat, if you haven't sanded it, use a grey 3M scuff pad to break up the gloss. It will ensure that the finish is able to 'bite' to the primer. Google some basic techniques for auto body. It's all about 'slick' and getting the surface very flat before shooting the paint or clear.