Only about 4 actually. I usually keep them until their batteries burn out and then they have to be replaced. I'm on an iphone 4 now and just haven't seen enough in the iphone 5 to want me to upgrade yet. I'm holding out for the next Nokia Lumina.
Have owned mobile phones since I was 19. I was unlucky with my first, it was stolen a month after owning it. Another one was gifted on my 21st - that was my third. Easily say 10 - three iPhones, and the rest were Nokia's.
3 - one for high school, one when I first came to AU and got locked into a contract, and the current one which is not on contract. I tend not to change between phones due to the hassle of data transfer and not to mention the $$$.