One of the tenets from a software-craft site I've been building. The idea is to keep state and dependencies as local as you can, and only promote something to a wider scope (module, shared, then global) once it's actually used widely enough to earn it. Most of the 'shared' code I've had to untangle started local and got hoisted up a tier too early.
squaresinger's point matches what I've found. Once three agents are going, you become the coordination point - you're holding the plan and reviewing all of it, and that part doesn't scale the way the generating does. What's kept it manageable for me is treating each one like an intern on a single, well-specified task I can check before it moves on, rather than running a swarm and hoping it converges. Wrote this up here: https://prickles.org/tenet/the-intern-pattern/AI1