If not, what would you predict?
I don't think there is any official figures, but I would definitely say the Imperials. Having a continent spanning empire is great for your birthrates, and the Cyrodiils have had three. I would guess that Mannish races would reproduce quicker then the Mer, as a result of their shorter lifespans.
Yeah, in The Real Barenziah books that's how it's described, Dunmer being an example, it's rare for Dunmer woman to conceive more than once in their lifetime, and humans are able to impregnate elves a lot more frequently than other elves due to their shorter lifespan. Such was the advice that the particularly promiscuous Barenziah was given to avoid having such encounters with humans too frequently and instead find someone of her own race before an accident might occur.
I'd rule out Elves for a start. Beyond that, I'd go by native provinces. Cyrodiil looks like it would produce the most food, so I imagine Imperials would have the highest population. On a related note I find the ESO idea that the race that doesn't go in for growing crops has the highest Elven population rather weird. I'd have thought that would be the Dunmer.
I don't buy that they normally only conceive once. If that was the case their population would very quickly decline.
Indeed. On Star Trek Voyager, they made that mistake with the Ocampa. Supposedly, they have a stable population, but if each woman can only produce one child, then their numbers should be halved each generation. Of course, http://tvtropes.org/pmwiki/pmwiki.php/Main/WritersCannotDoMath.
More on topic, I would expect the most numerous race to be the Imperials, before Oblivion, anyway. They have the largest home province and their forces occupied every other province for nearly four centuries. Post-Oblivion Crisis and Great War, it's harder to call. The Red Year probably takes the Dunmer out of the running, but it's hard to tell how the other provinces are doing relative to each other.
I just meant rarer more than once, not only once - although you are right, I went and checked the source I was going off of by memory, and it is a little different than what I said above.
"It is her child. Children are few among elves. No woman conceives more than four and that is very rare. Two is the allotted number. Some bear none, some only one. If I take this one from her, she may not conceive again." - The Real Barenziah
Also, just because I'm on the page, here's the quotes for the other stuff I mentioned.
"You appear to be with child, young as you are. Constant pairing with a human has brought you early to fertility."
"You mean promiscuous, although some do become prosttutes, I suppose. Elf women are promiscuous when they're young. You'll outgrow it. Perhaps you're beginning to already," Katisha said hopefully. "You ought to meet some nice elven boys, though. If you keep on keeping company with Khajiits and humans you'll find yourself pregnant soon."
Before the Red Year, sure. After the Red Year, I think it would probably the Altmer. It's hard to keep farming when half of your land is buried in ash and the other half is buried in Argonians. Not to mention how many of them died that year or how dependent their agriculture was on slave labor.
The Bretons probably but there's so little information on 4E High rock who knows.
I suspect the Dwemer may be the most populated race in Tamriel...
Which race is more numerous? I'd say that is close to impossible to say at all. We have no numbers on overall population, and the in-game representative percentages are notably unreliable since they don't count generic individuals, like guards and bandits.
What more is, while the Empire operates out of Cyrodiil and lots of Imperials migrate elsewhere(at least according to what is hinted at by the in-game demographics), there are a lot of non-Imperials in Cyrodiil too. It seems to easily be the most diverse country, with up to 75(ish)% being non-Imperial, at least before generic NPCs are added, at the time of Oblivion.
On top of this, I'd also say there is probably some pretty blurry lines whenver the Imperials are involved. Where does the line between Breton, Nibenese, Colovian and Nord go? Bretons and Colovians/Nords is easy enough to separate, but Bretons and Nibenese doesn't seem to be, both being inclined towards merchantile and magistocracy. Colovians and Nords are very similar too, "Shezarr and the Divines" going so far as to say the Colovians are "religiously, and resolutely, Nordic" and the Colovians even worship Shezarr as Shor. None of this comes across very well(or at all) in Oblivion, but the written lore supports it well enough("Shezarr and the Divines" being a book added by Oblivion, referring to Tiber Septim, so it is at least somewhat contemporary to the game). This makes me think of the "Imperial race" as generally redundant, since you'd effectively get the same result by having Colovia mostly populated by Nords and Niben by Bretons, but that's a discussion for another time.
In conclusion, I'd say that the most numerous race is probably one of the human races, but that is really as far as one can go.
According to UESP, http://uesp.net/wiki/Lore:Red_Year. Red Mountain isn't just a volcano, it's a super volcano that, when accounting for game scale, is probably at least as large as Ireland. Tear had major earthquakes and flooding, and there was even ecological damage from the eruption in Black Marsh. But yeah, two hundred years is a long time to rebuild and repopulate, even with the slower elven birthrate, especially since they didn't participate in the Great War.