Game Mechanics: Maximum amount of mobs able to melee at once

[Solist]

I distinctly remember this:

If you aggro’d 5 or more mobs, only 4 (the same four, I don’t know which order or how those four would be picked, likely first in melee range) would swing at you.

The other 1-50mobs would just stand there in a daze not doing anything.

Any cunning researchers want to have a crack at this? It was 100% a thing in this era and prior, I have vivid memories of kobolds in SolB just standing there until someone else was high enough on their hate list to switch targets, and resume attacking.