12 coats. You need to sand all layers you apply to keep smoothing it out so there are no big bumps at the end. And it needs to be progressive, mabey start off with some 260 grit for the first coat, and work up to some 3000 grit or 0000 steel wool for the final few layers. The automotive stuff need to be wetted before use but gives an amazing finish. Steel wool work good too, but any little splinters that get trapped in the varathane could rust and look bad.
You dont want to put it on really thick either, that could cause bumps or uneaveness in the final finish and make sanding difficult. The ideal finish should be many thin layers of poly. Thats why lacquer finishes look so good, they are applied as a very thin spray, and usually have 20-30 coats.