I'd recommend somewhere between 6 and 10. You can categorize them by your odds of getting in. At least one given (where you're almost surely accepted), a few probablys, a couple maybes, and a couple probably nots.
I went a little overboard on the probably nots and ended up just throwing away money. Each application costs around $70, so it was a big waste of money to apply to schools where I had a 2% chance of getting in.
A couple other things to consider. Any numbers / probabilities you see will always be outdated (unless they're from the year your applying, in which case they offer no guidence over where to apply because it would already be too late). Therefore you should be carefull relying on the odds. The trend right now is for the odds of getting in to decrease, not increase.
Secondly, think about how important rank is to you. If your barely getting into schools you're unlikely to get any scholarships, but if you're over qualified you can go to law school for a lot less money. I honestly don't think the education is that different depending on the rank of the school. There are lot of other factors (ie - which professors you get) that will make a big difference. Reputation is important in getting a job, which is probably something you want to do - but its not necessarily that higher ranked is better. Location, alumni, and specific reputation (ie "good public interest school") play important roles.
Okay - that was a little more than you asked for...