How to handle many bullets without lag and problem

Basically if I were to make a game with lots of humanoids npc shooting eachother kinda like AI wars in roblox but way bigger than that, how would I make it so that the bullets won’t lag.

some suggestion I found is that I have to render the bullet in cilent but that didn’t really make sense to me because hit detection and stuff, it would be nice if help explain
another suggestion is to use fastcast

if anyone got any suggestion please tell me below thanks in advance