According to wikipedia [1], the figure is 50,000. The peak headcount was 883,000, but due to mechanization, those 50,000 way out-produce those 883,000. Looking at the graph on that page, coal production is down but is still higher than what it was 50 years ago.
There are only ~80,000 coal miners in the US now, which is 0.06% of the full-time employee population.