I posted this in another thread and I'll say it again - a letter of intent is serious business - basically telling a school that you'll go there if accepted and will turn down any other offers. It isn't legally binding, but it is ethically. Sending more than one is unnecessary; they get the idea.
Now, if you're talking about a letter of INTEREST - then sending more than one is ok - but I wouldn't send 6 million or anything. Send one if you're on a waiting list, if you have something to update them on, etc.