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.

[1] https://en.wikipedia.org/wiki/Coal_mining_in_the_United_Stat...