Think this answers your question. (worth 2 minutes of everyone's time, or 10 if you had to re-read it 5 times like I did)
Short selling How Long does a Short Seller have before covering:
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Margin calls:
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Will be interesting to see what happens tomorrow, not sure how many shorts are left out there. Robinhood and the others that shut down trading may have given the shorts enough time to cover their shorts and avoid even larger losses. Melvin capital and Citron both claimed they were out of their positions yesterday, published all over Bloomberg and the like. (Not sure what to believe)