If you intend on breeding alot of guppies 20+ you will probably have to get about a 10G tank to keep them in, im breeding a moscow blue guppy with 2 females.
I have an 8G which houses around 16 babys, you can re introduce them to the main tank at around 3 weeks-1 month old
Depending on what male you areb reeding will also depend on the price you get for the guppies, you wont get much for females if any at all. But if you have a decent male as the father you should get something for the males

I swap mine into my LFS for store credit so i can get fish and aquarium equipment for cheaper
Here are some pics of my baby tank, its full of algae, but my cherry shrimp love it

, they have also had babies
