Made a sweep of Avatar fanfics in ff dot net.
Search says there are 42K while its section says 43.2K. I'm going to work with 42K since I used the search function for this and I wasn't gonna count variables anyway.
There are 42K Avatar fanfics on ff dot net.
At 25 fics per page there are a total of 1681 pages of Avatar fics.
The pairing fuction these charts use was implemented in October 2013.
Searching by publication date puts October of 2013 at page 354 of 1681.
354 pages that can be counted with 25 fics each equals 8850 fics that can be counted.
That leaves 1327 that can't be counted with 25 fics each being equal to 33175 fics that can't be counted
Out of the 42K fics 21% can be counted while 78% are left out, and that's quite a chunk.
So repeating myself: the charts reliably indicate the biggest groupings, but all the others are more than probably all over the place.