There is nothing wrong with having that many Overdieties.
Sure. When I say it's too many, I mean it's too many if the goal involves having the world follow D&D norms and precedents. If that isn't a goal, you could do whatever, even make all deities Overdeities.
Somebody is going to bring up how there isn't any "norm" because D&D worlds vary so wildly, from no deities (Athas) to a lot of deities, including a ridiculous number of greater powers (Faerun), etc. I don't disagree with that. I still maintain that there is consistency in that Overdeities are extremely rare and not even necessary for a world. Most worlds have either 0 or 1 of them, with Krynn possibly having 2.
No D&D world is split up into portfolios watched over by Overdeities. The basic concept of Overdeities is that they are beyond the concept of portfolios and normal deity stuff.
In fact we don't even know if such a concept exists in 5e do we?
I can't find a reference to Overdeities in the DMG or SCAG, so you might very well be right--they may not have even been mentioned in official 5e sources. I do remember Jeremy Crawford mentioning Ao in that recent Sage Advice podcast though. The 5e pattern seems to be that AD&D lore is the standard unless stated otherwise. Which is probably a good idea, seeing that AD&D ran for 23 or so years, and most of its lore was also preserved for the next 7 or so years of 3e.
The only god that has stats is Tiamat, and then we have a campaign that later stats the Demon Lords, who don't look any different than gods (if we use her as the template).
Yeah, it's rather unfortunate. They are also inconsistent about the nature of Demigods. The DMG rejects previous edition usage as the lowest rank of actual deities who can grant spells and such in favor of the literal meaning of a being who has both a mortal and divine parent, but other sources in 5e still refer to demigods in the traditional D&D sense. We have very little reliable info about 5e deities to go on.
Again, I'll use Marvel (not the movie production) Comics as the analogy. They have all of the mythological gods from every pantheon, in the Upper Planes (called Dimensions). Marvel has being that are even more powerful than gods, or skyfathers like Odin or Zeus, such as Lord Chaos and Master Order, Eternity and The Living Tribunal. Either one of them are capable of destroying the entire Universe, yet they are capable of working individually, without overlap/conflict.
That's fine for Marvel, but I'm going with D&D precedent.
AO is not the only overgod in FR, Fate and the Celestial Emperer (head of the Kara Tur Pantheon, but functions like an Overgod, not greater God). There is also whatever being AO was shown answering too.
I disagree with this assessment. Fate isn't necessarily even a deity. The presentation leans more towards it being a force, though the exact nature was (probably intentionally) left vague. I'm not sure if the Celestial Emperor from Kara-Tur's Celestial Bureaucracy is the same as the Jade Emperor from the standard D&D Celestial Bureaucracy, but the latter is not an Overdeity. Just like Zeus, Odin, and all the other heads of pantheons, they are a greater god, generally one amongst many (the Elven pantheon is odd in that only Corellon is a greater deity). Head of Pantheon rarely if ever equals Overdeity. Overdeities are above pantheons. Unlike heads of pantheons, they aren't worshipped by mortals, and their very existence is usually either unknown to mortals (as Ao was until the Time of Troubles) or known to the learned, but not relevant to mortal religion.
As a tangent, it's rather unclear how the Kara-Tur Celestial Bureaucracy is related to the more general one. They overtly share one or two deities, and there are a few more than might be the same deity viewed a bit differently, but then there are others that are just completely different. I tend to view it like the Elven or Dwarven pantheons. There is one Celestial Bureaucracy. There are a core set of deities known pretty much everywhere, and then there are some that are only well known on certain worlds. The Celestial Bureaucracy probably has a lot of deities (
Legends & Lore even mentions one or two in passing without giving them any other attention), with only a handful being universal, and the rest being known in a limited number of worlds.
Maybe at some point we'll get a more accurate and consistent explanation of 5e deities, preferably with a listing of divine ranks. In particular, it is unclear what to do with deities (like most of the Krynn ones) that have only ever been presented as Intermediate Deities, now that that rank has been removed (probably for the better).