I often hear news reporters say something like “John Young was only one of three people who have visited the moon twice”  or “she was only one of two people to have survived the plane crash.” What they mean is “John Young was one of only three people who have visited the moon twice” and “she was one of only two people to have survived the plane crash.”

The word “only” signifies that the number to follow is unexpectedly or unfortunately low or otherwise shows rarity. Of course John Young is only one person. How many people can one person be? It makes no sense to say he is only one in this context, since there are two others. The number in the first example that is low is three, i.e. the number of people who have visited the moon twice. In the second example it is the number two. Those are the numbers that should have the word only directly before them. This is one subtype of a more general rule that modifiers should come immediately before the word they modify.