They count all the computers authorized to play your content, not just share. Unless you explicitly deauthorize a computer, it stays authorized forever and counts against your quota of 5.
Unfortunately, unlike *association*, there is no way that I know of to determine what computers are authorized with your AppleID, so if there's confusion, the cleanest thing to do is..
In iTunes, go to Store->View my AppleID->Deauthorize all, then reauthorize just individually reauthorize the ones you need.
You can only do this once a year, so it's always important to deauthorize an obsolete computer before getting rid of it.