The S&P 500 Shariah includes only Shariah-compliant firms from the 500 largest firms listed in the US stock exchanges. You can download the Shariah-compliant constituents of the S&P 500 index to know the exact number of S&P500 firms that are Shariah-compliant. I will not expect any significant differences between the S&P500 index and S&P500 Shariah in terms of returns and volatility since S&P 500 Shariah is a subset of S&P500.