I believe the amount would depend on how cool you can keep the water. Minnows use less oxygen in cooler water. Even in the cooler the water will warm up if you open it a lot. It would also depend on whether you are going to provide aeration. If you just plan on using the cooler with water the same temp as the minnows when you purchase them you should be able to keep 10 dozen all day by keeping the water temp the same. I have done that in 4 gallons of water and keeping the water cool. I have also keep 10 doz. in a regular styrofoam 2 gl bucket with aeroation and keeping the water cool.