5 posts Don't need too much cause there are minority of visitor will actually scroll all your page though Anyway, the more the better for Adsense maybe? Cause you will have more keyword...
I think I have 7 currently... but I also try to limit the amount of the post the is shown. If you don't you the "more break" (or whever it's called in wordpress) and like to have full long threads on the front page I would limit it to 3 or 4... if you keep things short on the index, a bunch 10-20 could be cool.
I think it depends on how you show them on your index.