None. I have the same that came with my phone (an iPhone 5s I've had since Sept. 24, 2013), and I also still have the original one that came with my iPad mini as well (I've had since Nov. 2, 2012).
Zero... I'm using the one that I've had since the iPhone 5s was released. I've never had to buy an iPhone charger in my life and I've had the past three iPhones.
It isn't that hard to care for a charger. What do you do with it? are you buying authentic, Apple chargers or cheap ones?
I've had an iPhone since the beginning of '11, but I have no idea how many chargers I've had since then. But the last iPhone I got was my iPhone 5 in 2012, I've had 4 or 5 since then