RE:Marketcap Calculation (with link)Total shares outstanding is part of the companies obligation to report and is tracked by the exchange it is listed on. Regardless if the shares are sold into Long Positions or used in a Short Position trade, there is only one source for the shares.
If you are thinking short shares are getting lent out multiple times by the same broker then that presents a very unique opportunity for Long position traders as the process of lending shares short repeatedly creates what is called a virtual float. The shares still need to be bought back (covered) however 1 short share could result in 2 or more shares being required to address the short position. A virtual float (in theory) can exceed actual float and result in jaw dropping increases in share price once either market mechanics force the covering process to occur or if the company releases strong positive news.
The above was illustrated in action during the gama-squeeze experienced on Gamestop. There are videos and movies out now that explain the occurrence in more detail
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