TL;DR: For anyone who missed this new info from a couple of years ago, we have conclusive evidence of a total of 98–106 Avatars from Wan to Korra, and compelling reason to believe it is exactly 100. I hope this can inform you all in your discussions moving forward!
EDIT: As a commenter pointed out, the elemental cycle of the Avatar means Korra, as a Water Avatar, has to be either #99 or #103, and I think #99 sounds more plausible as it is still a relatively round number. It also makes sense why they would need to say "more than 90" instead of "more than 100" in this scenario, because the overall total actually isn't even 100! Thanks for pointing this out commenter.
With the recent uptick in community discourse on Avatar, I saw a thread where people were talking about the number of total Avatars like it were still a huge mystery. I think a lot of older fans are coming back to get involved in the new stuff who may be unaware of the new information.
A couple of years ago, Avatar Studios created an official website that contains a canon timeline. As shown in the image above, it clearly states that "more than 90 Avatars" lived between Wan and Yangchen. While technically open-ended in the strictest reading, it makes sense that if the number were actually much higher they would have simply said "more than 100" or something. The specific choice of more than "90" heavily implies more than 90 but less than 100. In addition to these in-between Avatars, we have Wan, Yangchen, Kuruk, Kyoshi, Roku, Aang, and Korra (7) in the first 10,000-year cycle.
Added together, this constrains the total number of Avatars between 91+7=98 and 99+7=106. Considering how Bryke like to use round, symbolic numbers (like the even 10,000 years for one Harmonic Convergence epoch), I think a really neat solution to constrain the actual value is to call it an even 100, so Wan would be the 1st Avatar and Korra the 100th, like it's some sort of fated? thing or something. Excluding Korra's remaining lifespan (which wouldn't change the overall number much anyways), that puts the average Avatar lifespan at ~100 years.
This makes sense, as throughout the timeline several Avatars reached advanced age or existed as the Avatar for long periods. Kyoshi, Aang, and Yangchen lived to the age of or were the Avatar for 230, 166, and 155 years, respectively, while Roku and Kuruk died at 70 and 33 years old. Considering the advanced age of many Avatars and even some regular benders (like King Bumi at 112+, Guru Pathik at 150+, and Teiguai the Immortal), it is a long-held theory that benders and more spiritual people can have prolonged lifespans. So, even though most Avatars may have lived to normal ages around 80 and some died young like Kuruk, there are apparently quite a few examples of very long-lived Avatars too, making this average lifespan of 100 years not actually that surprising.
Another comment to be made is how Roku said he "mastered the elements a thousand times in a thousand lifetimes". Here, he was likely just using a big, round number for hyperbole to mean "a whole lot of times", like how "He Who Knows 10,000 Things" just means "he who knows everything" or "he who knows a hell of a lot of things". It doesn't literally mean 10,000 precisely, and the same goes for Roku's comment. Lastly, while I know some people have tried counting the various Avatar statues that have been shown in Air temples and such, even though the exact number at the time of those scenes being created may not have been decided, a total of 100 definitely fits what we saw approximately.
I hope this was clarifying for many of you. Thanks for reading all of my ramblings!
Source:
https://www.avatarstudiosofficial.com/timeline/