There probably was less than two hundred avatars by korras time

The shows make it seem like there was like thousands but it’s 10,000 years and we assume most lived to around 80, which is being conservative, there would have been 125 avatar.

For a real word comparison, there have only been 14 dalai lamas starting in the late 14th century, who are similarly recognized at a young age

And we know kyoshi lived to be like 200, so who knows how long most avatars live. Kuruk died young, roku died in the volcano, and aang was technically like 160 when he died

So potentially there was only a few dozen avatars before korra