I assume this question is specific to the US because there is no reason anyone elsewhere in the world would not have access to insurance if they want it.
Millionaires got that way by being sensible with money, which includes insuring catastrophic, unpredictable risks. Assuming we can define a millionaire as someone who has a net worth of one million dollars or more, one of the surest ways not to be a millionaire anymore is to pay for a catastrophic medical need out of pocket. The amount of money needed to effectively self-insure all potential medical needs is huge. I would probably want to have at least 5 million in disposable funds for potential lifetime medical expenses PER PERSON/FAMILY MEMBER. Without a net worth of more than 100 million, I wouldn’t even consider it, and even then it would be a dumb financial decision since you could easily afford the most expensive Gold plan on the ACA exchanges if you don’t qualify for a more bespoke but underwritten individual plan otherwise.
As a practical matter, most probably have insurance as nominal employees of their company or foundation, or, for wealthy Hollywood types, through their (actors’, directors’, writers’, etc.) union or guild. Many of the rich probably use “concierge”-type providers for routine care, which don’t generally take insurance and only dip into insurance coverage for major expenses.
Served as a juror on a 1st-degree murder trial