I think for most models, I usually end up ~10 colors, including washes and basing. My painting skill is not tremendously advanced, nor do I have a large skillset. I usually just "color in the lines", and then douse stuff in ink wash.
For the Grey Knights I just
finished, I think I ended up at 11 for most of the models.
Black primer
Leadbelcher base
Runefang Steel layer
Mephiston Red - details/capes/wax seals/etc
Shining Gold - details/hilts/pauldrons/etc
Bone - details/scrolls/skulls/etc
Privateer Trollblood Base - Blades
Privateer Underbelly Blue - Blade shade
Privateer Boostrap Leather - Dirt/mud
Privateer Trollblood Highlight - Dirt/Mud
Agrax Earthshade - Wash