LOL, you only need one, but if you have a separate hatcher, then you can keep the bator clean and can have staggered hatches-you just remove any eggs on their Day 18 at the proper time and put them in the hatcher, which is ready to go with proper temp and the higher humidity. I incubate them in my Hovabator and use the homemade Fridge-a-Bator for hatching.