Overwhelm Points.

[Fiery106]

I know how Overwhelm works, but I don't really know what the threshold is for every tank. How would I know?

[Whitey]

"Champions will now get an "overwhelmed" debuff when more than 15 enemies are attacking them (using the same definition of "attacking them" as other effects). The debuff causes them to take 10% more damage from monsters for each enemy attacking them above 15 (10% at 16 attackers, 100% at 25 attackers, 200% at 35 attackers, 250% at 40 attackers, 850% at 100 attackers, etc.). Some champions have abilities that increase the starting point or decrease the debuff. This debuff shows up as a FA icon on champions who are being overwhelmed."

[Jerry]

https://steamcommunity.com/app/627690/discussions/0/1743353164079718808/

(just wanted to quote the same one as above)

To add, the thread lists also some exceptions like Tyril and Arkhan, who have a higher limit.