That depends...
The more colors that you use, the larger your image will be.
For example, if you said "Only use 4 colors, your image would be small...and ugly ;-) If you said use 64,000 colors, your image would be huge and have a lot of detail.
When you Save for web, you have the ability to use as many colors as you like to balance size against quality.
If it is a graphic, you can save it as a .gif format. If it a photograph or an image that requires higher quality, save it as a .jpeg image.
When you click on the Optimized version of your file, it should have selected the minimum colors that are necessary for the quality you have selected. I usually set my quality at 60.
Hope I didn't confuse you [confused]
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