This article explains it quite well. https://www.snopes.com/fact-check/643000-bankruptcies-in-the-u-s-every-year-due-to-medical-bills/
The long and short of it seems to be that the number is probably less than 500,000 a year, but that there are certainly that many every few years, at least. It's certainly not a good idea to get seriously ill in the US unless you're doing well financially, or at least have good insurance.