I applied to 4, but had completed 4.75.
I like my approach: one safety, three probables/possibles and one stretch (this one being the one that didn't quite make it to the submission stage). All were schools I would have attended. I think it makes sense to apply across a range of clusters: I know of enough applicants who didn't get in anywhere except the saftey school -- no shame in that, but they're probably happy they had that option. I definitely was not willing to wait another year.
Here's what I am peeved about: I REALLY wish I had applied to that stretch school (Stanford). An enormous stretch, obviously, but now I'm saying "What if?" Don't get me wrong -- I'm thrilled with the outcome I had. I just wish I knew how I would have stacked up.
I think my advice on this: only apply to schools that you believe you really would attend. And have a solid understanding of your profile and how you might (generally) stack up. I think some people overshoot and wind up disappointed (if only because of the volatility of the process). And I think some undershoot.
Oh lord, and give yourself LOTS of time. I whipped out 4 applications in 4 or 5 weeks. This was hell. I would have had an extra month, but ended up having to take the GMAT again.