We follow manufacturers’ instructions in order to achieve the best possible finish with the products we are using. Generally, one coat is not sufficient for achieving a nice finish. The exception would be a ceiling that gets sprayed. Spray application distributes the paint in such a way that one coat can be very thick, and on a ceiling it won’t run. On a rare occasion when interior walls are being repainted with exactly the same colour, one coat may be sufficient.
On the other hand, with some colours, and especially with Farrow and Ball paint, three coats are necessary. With gloss, an undercoat is a strict requirement - satin and eggshell are self-undercoating, so two coats are needed. For exterior work the number of coats is really important as more coats will offer better protection against the weather. We usually paint two coats of masonry paint on walls and three coats of an appropriate paint system on woodwork (this doesn’t include primer and stabiliser).