If 6 is the max for a 10G, I would think 12 is the max for a 20G. Not sure how live plants or other species would imapct that. I have 6 bettas in my 20G and the 2 clown plecos. I have gotten very lucky - my females are pretty docile. Even the gender confused male in with them.
If you are having aggression issues, try adding more decor - like big, bushy plants.
When I had silk plants, alot of my plants were like this and some were taller then the tank. Taller ones will float at the surface and provide hiding and resting spots for them at night.
Or you can add in some easy live plants.