20 gal for sure. Anything less and you would constantly have stocking issues with the gups breeding all the time. You'll still have them eventually with a 20...but it can naturally hold more fish so you have more time between dealing with removing babies.
I believe the question was in regards to what kind of a setup to do in a 20g? Was not the idea of this being a 20g already established?
All said and done, just make sure its heavily planted with good hiding plants. Or you could use a breeding box for whenever you notice a pregnant guppy.
