Good question.
To be honest, I don't know.
But, I suspect the answer is: "As many layers as needed to achieve what you want to achieve".
I recently completed an F-4 in splinter camo which required 4 different colours (layered), which was preceded by primer, then followed by clear coat, (decals) clear coat, wash, matte coat. So, in some parts, up to 8 layers.
Using an airbrush, I still managed to retain the details of the kit.
I think the key is to get nice thin layers down - for which i think an airbrush helps tremendously.
If you were to brush paint, the number of layers you can lay without losing kit details might be reduced.
In terms of the right sequence/materials, mine is as follows:
Primer (actual primer or just coloured paint), scheme colours, gloss, decals, gloss, wash, matte.
I normally use acrylics for everything except the wash for which i use enamels or artists oils.
Hope this helps.
Chris.