I guess about 20. But this is growing as I add more. Some of my clients have hundreds, but then that's why they ask me to write for them to keep them going.
My first response is... why? Seriously, I started on Blogspot, and when I realized how much work I had put into them and how much content there was on there, I couldn't bear to think of Google being able to just delete it all without explanation. It is much better to pay for hosting and have all that effort remain safe from Google's whimsy.
Currently maintaining 8 blogs in all (7 blogspot, 1 wordpress). That's too much for me since writing some posts for all of these is hard.. whew