3 ideas
A single counsellor decides difficult matters alone and unchecked, and can steer the ruler by his own interest. Two can collude against the ruler or deadlock in quarrel. Three or four check one another while staying small enough to keep secrets and reach decisions, so they give the best balance of scrutiny and control.
A decision counts as properly deliberated only once counsel has settled five things: the means of starting the undertaking, the men and materials available, the right place and time, the remedy against failure, and how the work will be completed. Taking counsel means walking every proposed action through all five, so no decision is made on the goal alone without its method, timing and fallback.
Counsel must be held in a secluded place and kept from anyone who does not need it, because a plan that leaks lets rivals anticipate and ruin it. Leaks come through careless talk, through the behavior of those who know, and through changes in their gestures and expressions. Secrecy therefore protects the entire time between deciding and acting, not just the meeting itself.