Should additional coats of polyurethane be applied after removing roughness?
Yes, the usual routine is to use steel wool or similar to smooth the results of the first coat, which raises grain in the wood. This will dull the finish. Then additional coats are applied, which don't tend to have the same effect, and which restore the luster of the finish. 

Two coats is considered the bare minimum by most folks. You could potentially buff a single coat to something satisfactory, but in my experience you'll have dry spots and a very thin protective layer with that approach.