We could approxomate the toatal from the real life equivalents.
Earth Kingdom (China): 1,357,000,000
Fire Nation (Japan): 127,000,000
Northern/Southern Water tribes (essentially two very large cities): 10,000,000 each
Air Nation: only a couple hundred, if that.
Add them all up and you get 1,504,000,000. You'll notice I leave out the URN. I included their population in with the Earth Kingdom. It's a rough estimate, but I think it makes sense with how much smaller the Avatar world is than our own.
As for the bending population, it's probably a fairly small minority, maybe 20%? (20% makes the bending population about 300 million total) I want to say that the rate was much lower in the Earth Kingdom, purely from looking at Ba Sing Se where Earthbending is rarely seen outside the military.
One should know what is the efficiency of agriculture in the world of Avatar. Since there can't be more people than those who can be fed.
However, in the real world of 1920 (approximately the tecnological level of ALOK) the world population was estimated in 1,857,000,000 of which approximately 1,000,000,000 lived in Asia (Siberia, China, India, Southeast Asia, Japan, Indonesia...).
I don't think Avatar's world (that's an "asiatic" world) to be bigger than Asia, that, in 1920 already included very densely populated areas. So I'll put 1,000,000,000 people as maximum limit.
As for the percentage of benders, I would estimate it between 5% and 10%.
Currently? I would mainly agree on the numbers. Still, there's the additional factors that:
a) Even assuming the era is approximately the 1930s, the largest city population in the US (New York) was only about 6 million or so (yeah, yeah, I know it's a bit of a biased point, but there doesn't seem much information otherwise, though I should love some links). Even if it's the main population of the country, it doesn't seem entirely likely that the whole population would live in one city. There are bound to be outskirts or other settlements, if not then, then at least now.
b) The population of benders was given as a statistic earlier on, though I can't cite it. The Fire Nation has (or had) the highest number of benders. The Air Nomads had the highest number of benders for their size, as all of the population could bend; the new Air Nation probably has about a 20% population of Nomads, with the rest as Acolytes. Then the combined Water Tribes, and finally the Earth Kingdom, wherein may be found only a couple of benders per town. URN? No idea.
The Creator of T.O.K. wrote: The simple fact that America (basically) opposes communism (at least, on a CJVK level) grants me the right to proclaim acts of genocide in the event that having multiple offspring becomes illegal.
Firelord: "Did someone just say...GENOCIDE!!?"
Someone with a glider: *Triggered*
Me: "Nnnnope. Not me."
Alt-Bending group. PRODUCE MORE BENDING BABIES #BenderGenocide