Right now, the five biggest companies in the S&P 500, Apple, Microsoft, Amazon, Nvidia, and Alphabet (both share classes) are 21.0% of the value of the S&P. Usually, the top-five share is lower. But it's possible that returns to scale are higher than they used to be, and that there's a secular trend towards larger companies.

For the purpose of this question, I'd treat multiple classes of stock as representing the same company (and, in the edge case where the biggest company has tracking stock, it's still going to count as one company). But spinoffs would not count, e.g. if Amazon spins off AWS, we would not add the value of the two surviving companies.

This question is not asking if today's top five will collectively be 21.0% of the market, just if the top five will have that share.