Quick answer
Nothing large should be attempted alone. The verse says it about strategy in general and then about the highest-stakes case available.
Read both lines
The first line makes counsel the thing that gives a plan standing, using the verb for being set firmly in place. The second line then picks the most extreme example the ancient world had, presumably because a war is where the cost of a private decision becomes unmistakable. The escalation is the argument. If advice is worth taking before you go to war, the smaller decision you were about to make alone is not the exception you think it is. This sits well beside verse 5, which described counsel as deep water in another person: one verse says it is there, this one says go get it.
Related: Proverbs 20:5 · Proverbs 15:22 · Proverbs 24:6 · Proverbs 11:14
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.