entity count

an entity counter for Minecraft 1.13-1.20 to list all currently loaded (vanilla) entities throughout your world (including all dimensions if the server allows it) in a scoreboard.

/function count:count

additionally, in versions 2022.1230 and above an "entity logging" feature exists which can help to find areas with large amount of mobs. unlike the counter, this has support for modded entities.

/function count:log