Apple admits that its some iPhone 5s have manufacturing issue that’s causing reduced battery life, and gave a valid statement to the New York Times-
We recently discovered a manufacturing issue affecting a very limited number of iPhone 5S devices that could cause the battery to take longer to charge or result in reduced battery life. We are reaching out to customers with affected phones and will provide them with a replacement phone.
However, its still not clear that how many iPhones are having these problems, but according to the New York Times it’s a few thousand keeping in mind that, Apple said it sold 9 million new iPhones the weekend the iPhone launched.