Ok, time for the standard stocking chart I got off the net years ago;
Newly free swimming fry 40 fry per gallon
Two week old fry 20 fry per gallon
Month old fry 10 fry per gallon
Pea size bodies 3 fish per gallon
Dime size bodies 2 fish per gallon
Nickel size bodies 1 fish per gallon
Quarter size bodies 1 fish per 2 gallons
Silver Dollar size bodies 1 fish per 3 gallons
Potential breeders 1 fish per 5 gallons
Show Specimens 1 fish per 10 gallons
One breeding pair 20 gallon high tank
You can see by this chart that a 55 will be fine for 5 or 6 show size angels. 18 dollar body size will fit in a 55, I've had plenty of angels smaller than that pair off & spawn. In all actuality you could have 10 in there long term with proper filtration & plenty of water changes. I would be surprized if you didn't get any pairs with 10, but they will stake out their territory when spawning, any aggression will be spread among the rest of the fish, but it will work.
I've had a 55 stocked with somwhere around 18 angels for better than a year now, a buddy of mine has had a 55 with a dozen or so adults for going on 3 years. Remember, these are bare bottom tanks, a couple of artificial plants to make it a little easier on the eyes, and that's it for decorations. Tossing in a couple inches of gravel, some large rocks, and a ton of plants & other decos turns your 55 into a 45.
You also have to remember that angels are new world cichlids, and have the attitude associated with n.w. cichlids, though considered semi-aggressive. Some are laid back, others are incredibly brutal, not only to other angels when they have paired, but also to their mate. Any time you are dealing with cichlids you have to have a divider or a spare tank handy, along with a backup plan. There are no guarantees with cichlid behavior.