If you have too many panties, that's easy. Just sort through the panties left in front of you, and find enough to throw away until you're down to your number. Don't forget you've already got 5 in the drawer. So if your number is 12, you only need to find 7 more to keep out of what's laying in front of you.
There's no specific rules about which ones to keep or toss. You could throw out the oldest ones. The least comfortable ones. Or the least sexy ones (that's what I'd recommend). But sort them out however you want, and make sure to keep yourself buzzing the entire time.