Isn't it "The Adventure of the Reigate Squires"?
Actually, this one's had several titles, depending on which edition you're discussing.
In the original UK eds., it is indeed 'Reigate Squire', singular. Most Sherlockians assume this was a printer's error, and it has since been generally corrected to the more logical plural.
But in American eds., the story is known as 'The Reigate Puzzle' or sometimes the mildly redundant 'Adventure of the Reigate Puzzle'. (The publishers figured that those provincial Yanks wouldn't get the concept of 'Squire'.)