Well, there is a transition at something around 20, and another transition at larger numbers too (no idea where exactly, but 500 is above the transition).
I find it interesting that everybody focus on the transition at 100. (Except for educators, that are completely aware of the 20 one.)
Quite useful in that context, as you say, like a mental model.-
PS. Interesting that the "threshold" seems to be somewhere between 80 transitioning to 100 (ballparking, of course).-