I got an email from Adsense telling me to adhere to quality guidelines but they only singled out one of my sites (while telling me to review quality guidelines for all my sites), so I removed the ads from that one. But what about my other sites? How many warnings do you get before they slap you with a ban?

I received such warning before, when i was doing some hardcore blackhat (pages with no content just ads). I do not think you receive two of them so be careful.

Don't practice any extreme blackhat methods on pages you have adsense on...