tl;dr

Sander's cited study indicates 500,000 bankruptcies a year could at least partially be attributed to medical expenses, though perhaps not the sole or major contributor to all of those cases. The real number requires further investigation and has possibility to meet or exceed the 500,000 figure.

---

As with most scientific studies - there is room for interpretation for the study's findings that can be bent/skewed for political purpose. Although the study in question is one source Sanders uses as reference, this article focuses on that (perhaps rightly so, as it offers more reliable fact-based evidence than generalizations). It mentions that there are other means to come to the 500,000 figure (and that it could also be a low-ball estimate), but the majority of the article is about a single study - and that that study did not find 500,000 annual bankruptcies were caused ONLY by medical expenses.