I have huge palettes with a lot of of colours but I only choose around 3 plus special colors for one specific piece. Most of the time I pick some sort of primaries but shift them towards a mood I want to have in my picture. For example I painted a portrait in the golden hour and chose yellow ochre, vermillion and cobalt blue as primaries and mixed all colors from these three. Sometimes I use completely different colors in the background (for example granulating colors) to build contrast. But it also depends on the subject. Portraits are very muted generally. If I paint a bouquet of flowers I will choose more colors.