Estimates vary by data source and definition, but most land in a narrow range.
Based on Federal Reserve Survey of Consumer Finances data, roughly 2.5% of all Americans have $1 million or more in retirement accounts like 401(k)s and IRAs, while about 3.2% of American retirees specifically have $1 million or more in retirement accounts.
Some estimates run higher when counting total net worth rather than just retirement accounts, putting the figure closer to 8 to 10%. So the honest answer: somewhere between 2.5% and 10%, depending on what counts and whose data you trust, with 3 to 5% being the most commonly cited range.