I am not sure where the estimate of 77m shares based on. Currently TSLA is already about 51% owned by institutions. Compared to other popular stocks such as aapl, jnj, mmm, amzn, msft, .... they are between 60%-70% institution owned. Taking the mid-point, there will be about 15% of TSLA shares to be forced owned by institutions after S&P 500 inclusion, equating to about 28m shares. Do I take the wrong equation?
It has nothing to do with institutional holders. Currently S&P500 funds hold zero shares of TSLA. When Tesla is added to the S&P500 they will have to buy shares equivalent to ~0.8% of their total holdings. (Which amounts to about 77m shares.)