In general, only two coats of plaster are needed to have a good result. A base layer and a durable topcoat. However, sometimes a third coat is needed to smooth out any blemishes and imperfections.
Putting plaster on plywood is not a good idea. Plaster is a cement type material and will not stick to anything that has moisture in it. Drywall and PVA are ok because they do not retain moisture. But plaster on plywood can cause dry rot and will be uneven around joins in plywood panels.
For even a small ceiling or wall, it can take a full days work to get a good result. Up to three days for a large ceiling. There are a decent number of steps involved to mix and apply plaster, and some waiting, also. It’s always best to call for a quote and get a