I agree with Jay about the origin of the bumps. No matter HOW much cleaning & precautions, its going to happen. I have read about "extraordinary measures" to prevent dust, such as covering florescent lights with plastic , so they wont vibrate, etc, but there are just too many ways for dust to find its way onto the surface. I use a worn-out piece of scouring pad to scuff-sand in between coats.
How many coats is a matter of whether or not you thin the varnish. I'm a thinner. I use Schooner Gold varnish, because it has a high solids content at a low viscosity. The thinning makes it easier to apply light coats. With SG, I do the first two coats thinned 50% with 333 thinner, then 25% for subsequent coats. On a new build, 4 coats will do it, on a re-varnish, 2 or 3.