He has a companion video about that - apparently that particular SEM is the smallest in the world, and the manufacturer loaned him one to make videos about. But he had a ton of issues with it due to being at a high altitude.
How could high altitude negatively affect the machine? The machine uses a vacuum pump. High altitude should help the vacuum pump work faster and better.
This video says 1 per square meter per year. https://www.youtube.com/watch?v=0xD7SvtPAMM