There are little foam blocks that keep the display assembly snug to the frame circled in the pic below. 18 sounds like a lot though.
Other than that, if they are little square metal pieces they are likely the tabs from the bezel on the digitizer. Most of them, unless you are SUPER careful, will break off when you pull the screen out. When you get a new bezel/digitizer (I hope there was something wrong with the old one, which is why you took it apart) it should have fresh metal tabs on it.
Let me know if that helps.
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