In my opinion :
- natural (so, not controlled) fertility is basically a function of threats and resources : more threats = higher fertility rate, more resources = higher fertility rate. I know there are a few biological exceptions to that, but they are just that - exceptions.
- elves are biologically immortal, but external threats remain, and Kulthea is a very dangerous world.
- resources vary depending on the location, but also how efficient the resource gathering and management is.
Elven fertility rate should likely be lower than that of mortal races (because, you know, immortal), but not that much lower, because 1) external threats are of paramount importance and 2) resource management is often at least as efficient, if not more.
So the remaining factors are
- available resources (raw)
- cultural factors
And I think that is the point where elves differ from humans much, and even some elves (Iylari) differ from others (Erlini, Shuluri).
For instance, I see most erlini as not caring much about being immortal in theory, because the risk of death from external threats is major. They live long, yes, but the pressure to replace casualties is always there. On the other hand, their idea of resource management is likely a limiting factor : they do not overextend, they do not pressure the land for higher yield, and they will most likely not expand by clearing woods for increased farming surface. Resource availability becomes a factor, but one that is dictated by culture rather than by raw availability.
And it is easy to see how those factors can vary for other elven races - especially Loari and Dyari, for instance, who have a very different notion of "resource management".
The other factor, which is in my opinion common to all elves, is fertility control. Because, let's face it, giving birth is dangerous AND potentially debilitating. And elves, who are very long-lived, rely on their own physical and mental capabilities to survive throughout the years. Long lives also make people think of themselves as individuals more and more. Giving birth is necessary for survival of the race, but it is not seen as a personal need - quite the contrary, it is often seen as a potential risk and a hindrance to individual survival. I think this trait would be especially pronounced among Iylari, leading to widespread use of chemical, magical, or technological contraceptives. In some cases, the "social requirement" of procreation might be an explicit part of the social contract ("once you have two children raised to adulthood, you can do what you want"), but in any case, I can see a complete dissociation of sex and procreation in the mind of elves - sex is whenever the fancy strikes you (which might be often), procreation is a serious affair because it is necessary for the race but dangerous for the individual. Basically, as an elf, you don't procreate as soon as possible and as often as possible, you procreate only when the situation is optimal for survival and prosperity of offsprings, because you don't necessarily want to have many, so you want to make sure every one of them counts.
Depending on cultural aspects (male contraception, overall availability of contraceptives that can be used repeatedly without side effects and are consistently reliable), I can see a much higher part of the population considering same-sex relations as a necessary part of life. I can also see a case where males would not care that much if they procreate with a "lesser race" because the child would not be theirs to bear, literally speaking (elves are definitely not saints).
Then there is the issue of inter-racial procreation, but it is a full can of worms by itself.