Hi bud..
While different weather conditions, compressor psi, and thinner to paint ratios will change things a little, generally, 1 tin will usually be more than enough for large coverage areas. But to be honest, I always like to safeguard myself, for purposes of testing colours, and if I make a mistake here and there mixing thinner, or ratios of different colours. So as a general rule, I like to have 3 tins on hand for the bigger colours (greens and reds), 2 tins for medium needed colours (greys and blacks), and 1 tin for the colours with only small coverage areas.. Peace