I wish I had the room to do all 20 at once. I have a 2’x2’ grow tent - can fit 2 in there pretty nicely - which - just growing for me, so it works out about perfectly. The stink factor, dear God. Starting about 6 weeks in the whole house smells like it. I don’t notice it because I’m used to it, but if I’m gone for a while and come home I’m like “Holy hello white widows!”
I don’t use carbon filters or ventilation or anything. Literally running a 10" fan into the bottom - it circulates the air up through the top pretty well and keeps them happy. So if discretion is necessary, look at investing in carbon filters or proper ventilation of some sort.
Luckily, nobody ever comes over so I don’t have to worry about it much.
What kind of grow space do you have?