The numbers are almost zero:
Surprisingly low, in fact, considering there are usually about 20% of people who are outliers. But you can always find anecdotes to support anything you like.
I don't know a single Canadian who went to the US *for* medical care (as opposed to falling ill or being the victim of accident or violence on a visit). I do personally know one American family (originally from SoCal) who drove up from CT (a 9-10 hour drive) a couple of weeks ago partly to take advantage of their not-yet-expired Ontario health cards, as unfortunately their business has gone to sh*t. And good teaching hospitals get international patients all the time, whether funded by insurance, self-funded, or by charity (eg. the Herbie Fund).
It's hardly worth arguing though, there is far too much well-funded reactionary propaganda floating around, and it will only get even thicker and yet more tedious if there is any serious possibility of a change.
Best regards, Spehro Pefhany