But in the example presented, the choice still determines when the players find the mcguffin, and where they have the magic water to help then defeat the guardian.For me, unless my choice matters, it's not meaningful. If I choose A but that choice is irrelevant, i.e. no consequence, then it's not meaningful. So one. My answer is at least one consequence.
If you're using Quantum Ogre, you are, by definition, removing their choice. So there's no choice and therefore no impact of or on their choice. Tunnel A has the McGuffin. Tunnel B has the magic pool. The players choose A. Unless you let the players freely decide A and follow through with that choice (presenting them with the McGuffin), it's railroading.
the fact that I put the ogre in the way regardless of the decision does not remove all meaning from choice, and since you defined a single meaning as enough make the choice meaningful…the quantum ogre in this case has removed 0 meaningful choices from the player