If Avatar Wan lived 10,00 years prior to Korra's lifetime, then how many Avatars have there been?
In the Avatar episode "Southern Air Temple" we see a chamber containing approximately 130 statues representing prior incarnations of the Avatar. These are tiered in 7 ascending rows plus a further 51 statues at floor level giving a grand total of 181 previous avatars.
In the film "Avatar : The Last Airbender" we see the same temple, but this time with 6 tiers, each containing 30 status (in 5 blocks of 6) plus a larger statue of the most recent avatar; Roku. This gives a grand total of 181
Given the similarity in numbers, it seems likely that this figure is intended to be the canon figure.
Hard to say, in avatar TLA when in the air temple and when talking to roku about avatar state you see hundreds of avatars, however the first avatar wan lived 10,000 years ago, and avatars live quite long (like Kyoshi who lived about 200 years) so assuming all avatars live between 100 and 200 years, 10,000 / 200 = 50 and 10,000 / 100 = 100, that would only be between 50 and 100 avatars, which would contradict what's seen in the air temple, the math doesn't add up but I seem to be the only person I know who cares
Considering there were 181 avatar statues in the southern air temple when Aang visited it in episode three, book 1. We can say that there are 183 including Korra. That's what most of us got so that should be a close approximation of the real amount of avatars there have been in the full 10000 years from Wan to Korra.
The problem with this is that we don't know the ages of most avatars, the statue of Wan is of when he was young. But at the end of his storyline we see him right before he dies, and he had grey hair in that scene. The temple estimate seems most realistic because there is a statue of the first avatar which suggests they know how many there are between him and Karra/Aang. It seems like the average age is about 55-60 which could fit, seeing as the job is not exactly "safe". There is also a possibility that lots of avatars died at a early age.
When Roku stated I have lived 10000 lifetimes, a lifetime is not always a hundred years. But if an avatar lived at all (for instance if an avatar died at twenty) that's still a lifetime. He has lived the life of every avatar no matter how short their life was. I say there have been 10,000 avatars but since some of them have died early then maybe it adds up to what he sees as 1000 lifetimes.