Lacquer coats

Kind of like asking how many holes it takes to fill the Albert hall...

Just enough to get a glossy surface when dry.

Since lacquer dries quickly, I find I can do it in one "coat" where a coat means a session with the airbrush. I may make a couple of passes, waiting a few seconds between passes. I am usually doing several different spots for decals, so by the time I make a pass at each spot I can go back and make another pass at first area, and so on.

I put three coats of lacquer over a flat finish yesterday and it still did not look like a smooth enough surface for decals. Also, if you apply lacquer in decal spots will there not be a different sheen to the model than if you had painted the whole model with lacquer?

After decaling you apply a matt clearcoat,  both to seal the decals and to create a uniform (flat) sheen.