Asking how many tones a painting should have is a bit like asking how many brushes to use or how long it should take to finish a painting. And the answer similar: as many (long) as it needs and probably more (longer) than you think. Of course this is a totally unhelpful answer when you're new at painting, so here's a more tangible answer.
I think a painting wants at least five tones. The three basic tones are: dark, medium, and light. Then add another two inbetween tones, one between dark and medium, another between light and medium. That gives you five: dark, not-quite-so-dark, medium, not-quite-light, and light.
As you refine the painting, you'll probably end up with further levels of tone. Don't stress over this or waste time eliminating mid-tones, but do check you've retained your darkest and lightest tones. Don't use your darkest tone immediately, but use your second-darkest. That way when you refine the painting, you can still make something darker.
Exactly how dark and light your darkest and lightest tones will be depends on what colors you're using. The darkest shade of yellow is never going to be very dark, for instance. The range between your darkest and lightest tone can also vary, you need not always use the full range available but may decide your lightest tone is what would be a mid-tone in another painting.
Browse through the paintings from the Monochrome Painting Project, and you'll see how a painting can be done successfully with just one color, provided a range of tones is used.
See Also:
• Why is Tone so Important?
• The Three Sides to a Color's Personality: Hue, Tone, and Chroma
