As many kinds of beans as you can muster.
For this project, if your child likes to have a plan, you can suggest drawing a picture first, or print out a coloring page as a guideline. You can find a variety of printable coloring pages here. Then, have fun filling in the lines with your multicolored legumes. Squirt a little dab of glue, add a bean, repeat. Easy peasy.
If your child is not the planning type or doesn't like to be confined by lines or instructions or rules, then just let the creativity flow from their fingertips (as well as the glue bottle). My kids chose this route. Surprise, surprise.
For Finn, a huge glob of glue and no guidelines to fill didn't equal a lack of a plan. With each bean grabbed, he would state "this one goes here and this one goes here and this one goes here..." He seemed to know each bean's location before placing it on the paper.
And the result? A free form, but well thought out piece of abstract art.