Quick answer
Plans made alone tend to fail; plans tested against many voices tend to hold. The verse is about durability, not about taking a vote.
Read both lines
The first line names the failure mode as isolation rather than stupidity. Good plans collapse when nobody was permitted to find the flaw, because the maker of a plan is the last person able to see its blind side. The second line supplies the remedy in the plural, "a multitude of counselors," which the book repeats elsewhere with safety as the payoff (11:14; 24:6). Multitude matters because two advisors can share one blind spot. What the verse offers is not certainty and not consensus; the counselors "establish" a plan by stress-testing it, and the plan that survives several honest readers is simply built better than the one that survived only its author.
Related: Proverbs 11:14 · Proverbs 24:6 · Proverbs 12:15
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.