<p>I spoke to a carnegie mellon rep about this, and she said that it's fine to send 1 more than the requirement, "but no more." Also, it seems preferable if you send an additional letter addressed from a mentor of some sorts (e.g., from an internship) rather than a school teacher. They won't in any way look poorly on your application, that is for sure, but may simply only read one of them (they've got 10-20 mins on each application, depending on the school). Basically, don't sweat it -there is a TON of kids out there who don't follow guidelines and send 5-10 recs and mildly irritate the adcoms.</p>