There’s a lot of overlap.

In recent years, around 30% of newly appointed board members in the S&P 500 have been active or retired CEOs.

And:

CEOs and directors with financial backgrounds constitute 59% of the incoming class of S&P 500 directors…

Source, for whatever it’s worth. I admittedly just did a cursory Google search out of curiosity and that’s what popped up, but I had a feeling it’d be a significant percentage.