I never apply more than 3 - 4 coats in one session of clearing - and very rarely 4 coats. If I need to bury some graphics in clear I will apply 3 coats and let it dry, then sand the parts down, completely re-tape and re-apply 3 more coats.
Having said that - a job like this fire job was painted with the candy colors then cleared with two coats of clear. The graphic were airbrushed then 3 coats of clear was applied and sanded - to get the final glass-like finish 2 more coats of clear were applied before the set was sanded and buffed. SO you could say this set has 7 coats, but even that could be innacurate. As I am applying a coat of clear, the speed of my gun, the air pressure I am spraying at and the reduction of the clear can all make a difference. I can apply 3 coats of paint that will give you the same amount of paint thickness as another guy spraying 6 coats or someone else spraying one. You have a window of paint thickness measured in mils that many times is exceeded in custom paint, but that is more accurate than "How Many Coats of clear". So to answer the question - exactly enough! Depth is achieved by the way I spray the fire and shadow with mutliple colors, not by how many coats of clear. More coats of clear won't add shine. The finish is like glass - you can't feel any graphics, edges or lines and that's the way it ought to be if it's right!