It depends on the glass you are using. Tight weave 4oz, you may conceivably get away with one fill coat, but I would do two. Open weave maybe three, especially if you are rolling it on.

6oz glass rolled on you should consider three fill coats for sure. Maybe four. Depends on the resin you are using and the application method, also the temps in your workshop to some degree.

If you take a utility knife blade and use it as a scraper on a tiny section, you will soon see if you are cutting into the glass before you have got it smooth. Don't leave it too late though before over coating. You don't want to lose your chemical bond as you don't want to sand between fill coats, for obvious reasons.

If you were painting the hull you could get away with cutting into the glass here and there and go light in the fill coats. But an extra fill coat over the entire hull weighs probably less than 10oz and gives you a useful extra bit of buffer when you are sanding back, especially around the sheer, the chines and the stems.

My opinion.

Et