How to limit colors to only 24 colors?

I saw a program on a quilter who took a picture, made a mosaic and then was able to make a grid and quilt it. I opened it up in photoshop 7. I was able to make a mosaic, but I need to be able to limit the colors to (24). How would I do that? I would appreciate any information anyone could pass on to me.

Try Image > Adjustments > Posterize. Then specify 24 colors.

Art

Or convert the Mode to Indexed Color. Specify 24, and play with all the options to get the look you like best.