Invincibility +Def at 10 Mobs?

Correct. Although I'm not sure if it is even possible to actually get 10 human sized enemies in Invincibility's range.

Outside of Rikti monkeys, I don't think I've ever gotten more than 8 enemies in Invincibility's range.

In my informal testing, 8 was my max, too, usually with 1 or 2 mobs constantly fluxing in and out. I usually never count on more than 6-7.

10 standard human sized models can quite nicely fit in invinc's range. Enemies that automatically approach to melee, like cimerorans, don't even need any special coaxing (such as maneuvering or using a corner).

Invincibility extends slightly beyond a normal mob's melee range to 8ft, so it's not uncommon for 10 to be just within reach but are constantly running around trying to get closer to the player and never stopping long enough to put the count all the way to 10.