From the very sparce information that we have concerning the First Ones, one thing that we can safely assume is that they are the architects of the Warcraft Universe, makers of reality, they have establisher the order of the 6 main forces, set the rules etc.
In terms of power levels they are ultimate power, standing above the second tier of creatures - the pantheons of the cosmic forces.
Also, they are likely tending to be elusive and avoid active participation in any conflicts that take place across the Universe, even such ones like the Burning Crusade and destruction of the entire pantheon of the Arcane by defecting Sargeras.
Though now we can only speculate, having almost no information, let's discuss the following question - how many First Ones exist? One thing that is sure that there are more than one since it's plural. Let's look at the possible variants.
1. The Six. A pretty straightforward approach, I'm tending to bet on this one. Each of the First Ones is an entity that represents one of the main Forces and have a total control over it - Light, Void, Life, Death, Order, Chaos. Six mouths that hunger. In this case they've created the reality from the collision of their forces and established a balance between them. But that's a bit strange if since then all the 6 forces came to war with each other, and from mentions in Oribos we see that First Ones tend to strive for balance between the forces, not war. And also it seems that they together have established each force, namely Death, but in the case of Six I'd see it that each of the First Ones personally and solely orders his\her own domain. Also, their architecture seems to differ from the cosmic forces themselves. It the beginning they've ordered their forces and balance between them, created the Pantheons to rule them and then left somewhere, not interfering any further.
The pic I attach is the statue in Sanctum of Domination. Maybe it's not the case, but it resembles of a Grim Reaper, and if they'd ask me how would a creature that embodies and commands the Death as is its looks like, I'd think of something like a Reaper. So maybe this is a statue of the First One of Death.
2. The Three. Another option is that there are actually only three First Ones, and each one being dualistic in nature. So, the first one (of the First Ones
) controls both Light and Void, second one - Life and Death, third one - Chaos and Order. So they possess an internal conflict within themselves, being on the both sides of big conflicts. That theory has some indirect evidence. There's a connection, a possibility for a being of one force to turn into a creature of another force. E.g. naaru have numerous times turned into voidwalkers, and sometimes restored back. Sargeras, a titan of Order, has turned into a creature of Chaos. Also there's a connection between Life and Death via Ardenweald, with Elune being Winer Queen's sister. That could all be possible if the pairs of opposing forces actually originate from one being. That could explain the possibility of such changes.
3. The Two. Another possibility is that there only Two First ones, a dualictic scheme resembling zoroastrianism, with two supreme deities that define all the being, "Ahura Mazda" and "Angra Mainyu". The "good" deity commands Light, Life and Order while "bad" one controls Shadow, Death and Chaos. Still, this is the most unlikely option. The First Ones seem more like to cooperate, not wage war, to architect.
4. The Four. There's a speculation that there are actually 4 of them, and their number has no direct connection to the cosmic forces. This is supported by the idea thar they seemingly work together on each of the forces and have visual style that doesn't repeat any of them. It this case, though, they actually are a seventh force, that stands above all the other six. And that's quite another big retcon... probably.
5. Maybe more. If the 'lower' forces are also a part of the group - fire, water, air etc.
So, how do you think?