<p>I've only asked two so far. :(</p>
<p>Two is standard. The Common App gives two forms plus a counselor form. Some schools only require one, some don't require any, but two is a good number so you can have them if you need. I can't think of any that require more than two, although some people like to submit 1 or 2 supplemental recs. Admissions officers don't want a lot, though.</p>