1. Regarding the number of colors, it depends a lot on the look you’re trying to achieve. Also, using different shades of the colors for different pieces (moving left and right on the color scale) can give you rich and contrasting effects.
2. If you use two colors, a third, contrasting color really pops. (I’m not advocating using two colors instead of more, just making a point about visual impact.)
3. Take a look at a color wheel. "Complimentary colors" are called that because they are (complimentary). Shades of blue often look good with shades of orange, etc.