I think it works on larger tanks. As far as I know, you couldn't really put 2 in a 20g (certainly not one in a 10g). But 5 in a 50g sounds about right, but you could add other fish. I think it's more of a swimming space issue.
You could likely keep 8 or 10 in a 50 and not have any problems till more than one pair spawned.
I per 10 gals sounds generous to me. They have nothing like the body mass of discus, home of the original one per 10 gals rule.
With nothing else? You could have a lot, but once I got a shoal of ten I'd stop. Maybe I'd go 8, and assume no more than 2 pairs breeding at any point.
A pair of angels can fit in a 20 gal tank. My angel breeding tank is a 20 gal (24"x15"x15"). I've seen & heard a lot of 20 gal discus breeding tanks.
A tank around 4 feet can easily hold 10-12 angels, provided with regular water changes. In 10-12 probably u'd get 2 pairs atleast. But I'd recomend anything more than 2 pairs to be removed to a breeding tank.
Purple, it isn't really uncommon for 2 females to pair up. Happens with discus and angels. A guy in my local club has 4 angels spawning together and raising the fry together.
20gals is good only if its high and the angels are the only fish in there. Yes there are 20gal discus breeders but for the most part today...25-29gals are used.