Probably a lot? But it's honestly kind of hard to say and I have seen very conflicting data (such as the 4% vs 60% discrepancy in what percentage of bankruptcies are caused by medical bills). I do agree healthcare in the US is thoroughly terrible, even if for no other reason than exactly this uncertainty. If you go to the hospital or have some type of procedure done, you could be out a few hundred dollars or a few tens of thousands or even hundreds of thousands of dollars and there's really no way to know before you go in.
And my anecdotal experience only muddies my understanding even more. Things have always worked out pretty well for me. Even the time I got a $45,000 bill because the surgeon was covered by my insurance but the anesthesiologist was not, somehow in the end after a few phone calls to the billing department my portion was lowered to just an extra $400 on top of the expected copay. I have no clue how any of it works, but these insane numbers seem to often just go away as mysteriously as they appeared. I don't personally know anyone who has actually had to pay the absurd amounts of money that get quoted, it's like it's all some kind of fake money. (I do realize that I am probably in a very privileged bubble, most people I know are at least not uninsured for instance).
And my anecdotal experience only muddies my understanding even more. Things have always worked out pretty well for me. Even the time I got a $45,000 bill because the surgeon was covered by my insurance but the anesthesiologist was not, somehow in the end after a few phone calls to the billing department my portion was lowered to just an extra $400 on top of the expected copay. I have no clue how any of it works, but these insane numbers seem to often just go away as mysteriously as they appeared. I don't personally know anyone who has actually had to pay the absurd amounts of money that get quoted, it's like it's all some kind of fake money. (I do realize that I am probably in a very privileged bubble, most people I know are at least not uninsured for instance).