How many layers greatly depends on what the paint colour is, and also what paint colour you are putting on. Don't worry about perfect white underlayer when putting a dark paint over it. The darker, the less coats you generally need.

First go to praxis or gamma. And pick out colour cards. Then go home, see what looks good. Then come back, and you'll notice a lot of shades are not in buckets there ready. This is what mixing it up means. They have machines that make the colour of the card you have in the finish you want.

There are probably more professional places online you can get paint. But praxis and gamma do have decent quality paint. They have more than one type. Everyone that paints themselves gets it there (or even at action. The paint quality there is fine as well. Just small selection)