Years ago, McCain made a promise, as a result of some controversy or the other, to keep his wife's finances and properties separate from his own. As wealthy as she is, and as many companies as she owned, it was felt that the wise thing to do was for him to not even know what she owns, to help prevent conflict of interest situations. As a result, he had no idea how many homes she owned. If the question had been "how many homes do you own" it would have been an easy answer, but the question was "how many questions do you and your wife own" (may be worded slightly differently)
It should also be noted that they don't live in all of these. One is used for an elderly relative who apparently needed a place and live, another purchased for one of her children to stay in, and most are rental/investment properies, just like many others have. From what I understand, McCain himself only lives in two--one in his home state of Arizona, and one in DC.