Connor Rothschild charted all the colors Bob Ross used in The Joy of Painting:
Most commonly, paintings have 12 colors. Of the 403 pieces in The Joy of Painting, 100 used 12 colors.
The peak is concentrated around 12, meaning most of Ross’ paintings used somewhere in the range of 7-13 colors; very rarely did they venture outside of that range.
Grab the data here, which was collected by Jared Wilber.