The answer to this, for me, depends on what you're looking for.
If you're looking for a factory edge, which is typically around a 800 to 1,000 grit, you can get by with one stone (or one grit).
If you're after a little sharper, then you need two stones.
The problem is that if you ever chip the knife or it needs serious thinning, the 1 stone (which I'd recommend a 1,000 grit stone) will be a PITA to use to fix the knife.
For me, I would really like to step up to this:http://www.chefknivestogo.com/3pcstoneset.html
You can use wet/dry to flatten the 500 and use the 500 to flatten all the rest. It gives you means to fix/thin and get sharper than factory.