Fish Can Count Up To Five…

https://www.sciencedaily.com/releases/2022/04/220401122240.htm?fbclid=IwAR2_vBeE2V7USKKXuBW2_DdeQuOXuv053E3K45MPsjBl_f9ytFX92Cm6u10

When we keep 2 or 3 of a shoaling species it often doesn’t work, and they are decidedly antisocial with each other.
4 or 5 and they often just hang about not doing much, as if they’re “waiting for more”.
Keep “enough”, in the fishes’ opinion, and they’re flowing around the tank together and behaving as a group.

Maybe the magic number is simply more than they can count. Conceptually it makes a lot of sense…