Since the Affordable Care Act (ACA) health insurance marketplaces opened in 2014,there have been a number of changes in insurance participation as companies entered and exited states and also changed their footprint within states. The map below shows how insurance participation has changed from 2014 - 2018 in every county in the U.S. There are a number of areas in the country with just one exchange insurer. In 2018, approximately 26% of enrollees (living in 52% of counties) have access to just one insurer on the marketplace (up from 21% of enrollees living in 33% of counties in 2016).
Source: kff.org