OK first things first, when using a single stage there are different things that need to be considered,
First is what color and is it a metallic?
Metallic paints when used in a single stage are tricky to say the least, I.E. if you are shooting a base coat and you screw up like when you get blotchy spots you can just sand them down a bit and hit them again, single stages are not as easy to get rid of defects.
As far as how many coats you need again it depends on color, are you going to prime the whole car? if you are going with single then i would highly recommend it, if you are going to use a gray primer it will work good to get you a uniform look but if you are doing a heavy metallic or pearl color then you want to ask you paint suppler on what would be the best color primer to get the effect that you want, it is just a question of tinting.
Once you have your primer down and you have blocked it down smooth, to make sure that your body work is good, then apply you paint in nice even LIGHT coats, just take your time and overlap your spray pattern about 70 percent and keep a wet edge, as soon as it is covered and nice and even then stop, and when I say even and covered I mean look at it in all directions and in all different lighting, if you have some halogens that is what I would use, aim them down the side of the car and look at each panel lengthwise. (this also works really well when doing body work)
Hope this helps and good luck.
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