It is never adequately stated in the series or any other type of material. It seems to vary a lot depending on what the setting is. Sometimes benders seem like a gifted rarity and other times they are the norm. The Air Nomads – one of the four nations, is 100% benders. So one might say 25% of the world right there – although that doesn’t really matter now since they’re all dead except for Aang. Also, we don’t really know the population figures for each nation relative to one another – but that’s another story. The Air Nomads aside, the first Waterbender we meet is Katara, who is the only Waterbender in the whole Southern Water Tribe. However, can’t go by this example either since the Fire Nation targeted their Waterbenders and reduced their number.
In the Northern Water Tribe, the Earth Kingdom and the Fire Nation, there are no shortage of benders in the military forces. Most of the Earth Kingdom and Fire Nation soldiers we see are benders. There are some exceptions like the Yu Yan Archers, the Rough Rhinos, Piandao, the Kyoshi Warriors, etc., but overall benders seem commonplace here. Pretty much every Fire Nation or Earth Kingdom general is a bender. We don’t see any benders on Kyoshi Island or Chin Village – and not that many in the town where Aunt Wu was. The entire Fire Nation Royal Family are benders, but the Earth King, Hakoda and Arnook are not.
Based on what we know, it seems to be a combination of genes, spirituality and random luck. Non-benders Hakoda and Kya bore Katara, a bender, but two bending parents produced Piandao, a non-bender. When we talk about the spiritual factor, is it how spiritual the parents are that determines whether their kids are benders? I guess if Kya turned to spirituality due to the misfortunes her tribe faced that helped make her daughter the first bender in their tribe in 45 years. Of course, that also means that even if the bending population is exterminated in a given area, benders can still be reborn into their population. Whether the Fire Nation was aware of this when they raid the Water Tribes is unknown.
Since it is determined by birth whether or not you are a bender, the parents’ spirituality must be what counts, since the child’s clearly cannot by that point. That’s why people like the Boulder are benders and people like Guru Pathik are not. But how do they tell if you’re a bender or not. Piandao was abandoned at an early age, so they knew by then. Is there a test? Or do they just wait as you get older and see if you accidently start moving some rock or water around?
That’s pretty much it for now. I was going to touch on the subject of interbreeding between the nations and different types of benders next, but then I decided the post was long enough as it is.