I'm going to be honest about this, it hinges on the filtration you have and how much work you want to do also. filtration will only get you so far, water changing gets you the rest of the way.
if you want an easy time and you have a decent filter, 4-6 adult MALE angels TOPS. If you will have other fish with them and don't want it looking overrun with angels, go with 4 adult males, fill out with larger amazon tetra species, not huge ones but if neons they have to be the 1" to 1.5" max size adults or the angels will corner and kill them if they are juveniles and not fast enough yet.
More angels than this and the density gets rough, it gets expensive on food to keep them gaining mass,
nitrates build quick and water changes have to happen more often to keep the water quality up, if the water quality declines your first signs are fin fuzz and/or a cloudy eye, which will progress to more serious issues if the water quality isn't corrected.
Can't stress it enough, you can have mediocre looking angels with poorer water quality, or you can have pretty stunning angels with pristine water quality. So the easier you make it on yourself by not overstocking, the easier it is to keep them in top condition. A big school looks great, BUT it's a LOT of work to keep them fed and in top condition. A bunch of raggedy fish don't look good at all.
About impossible to identify males from females with certainty before maturity. I had luck rehoming females and buying quarter sized body juveniles to fill out again to get all males. and just kept 1 female on the side to breed to get to my group which i've whittled down to all males "I think".
If you go that route of unsexed angels, start with 6 juveniles, 1 or maybe 2 will be females, possibly 3.
Dime size bodies or smaller will get chased and maybe beaten to death with the mature angels wanting to eat them maybe at the nickel sized body also, Juvenile angelfish introductions to mature males, they really need to have that 25mm (1") body or bigger. That goes for all fish kept with adult male angels, they need to at least be 1" body plus fins to have a shot of being ignored as food. you can decide later to stay at 4 or 5, or go to six as they grow, or just 2 or 3, but seriously you have to stay on top of it, you can't leave females in there, and you have to watch for maturity and pairing or all heck is going to break loose and fish will get damaged.
I do not recommend more than 4-6 angels for anyone with any size tank above a 40 breeder and less angels for a smaller tank, 2 angels and nothing else, 20g minimum tank size, and that's pushing it.
Unless you have a couple years of experience with angels under your belt and have been through everything with them and then decide to do the work and be attentive to it. then you can make it as hard as you want. But I don't recommend making it harder than it has to be from the start until you get experience with them.
And I don't suggest going for 6 mature adults if you want to keep other fish with them.
I also recommend at least a 20g
QT/Hospital tank, they are bound to get injured from time to time they are cichlids, its going to happen, or a constantly picked on fish needing TLC to catch up. it's good to have flexibility. to be able to remove a fish at a moments notice like in the case of a female maturing, or one of them getting picked on too much and needs some time to catch up to the rest and de-stress. Even if you keep two juveniles in the 20 and 4 in the 55 and move fish around from time to time to keep it cycled in both places and everyone knows everyone and used to the switching around and fish coming and going. it kind of breaks them of the "mine" habit and becoming more sharing and less dominant.
You may find, that 4 or 6 fish just don't work for you because of the individuals involved, between dominant males and females maturing, but for some reason two fish get along like peas and carrots, or 3, and you might decide to rehome the ones that are difficult and just keep two or 3 males that aren't trouble.. or for the size of the fish and how you want the tank to be with the other fish, and how you want it to look or to take care of the tank, decide to go with just two or 3, so it's not so much work, or not Angel dominant. ,Also why I suggest that 20g "flexibility" tank. No harm no foul doing that. If you don't need it, you can always take it out of service. but it's a lifesaver when things aren't working out and you have to separate someone until you can rehome it.
best of luck. you can always add more, but start easy and get the fish you want first, add more or less so it's how you want it to be. there are mild mannered angels out there that just follow the herd their whole lives, and relatively easy, there are some really ornery ones also, it may take some time to find the right fit for you and your tank. that works for everything long term.
you have options to choose from and they're all perfectly fine. Depends on what you're looking for in your day-to-day enjoyment when watching your tank.
