The US, like many countries and regions, has poor coverage of abandoned, closed, and shuttered mine sites despite such sites still posing an ongoing danger in terms of imminent physical danger (collapse, decay, etc) and untreated waste piles and ponds leaching toxins into ground waters, etc.

To answer the question posed, "how many (US?) mine sites pose a danger of type {X}" requires crawling the US BLM datasets, the OSHA datasets, the archived (from when active) MSHA datasets, and having a some luck onside for various specific sites due to large gaps and periods of not caring at all.

See:

* https://www.epa.gov/epcra/does-msha-have-jurisdiction-over-i...

* https://www.blm.gov/programs/aml-environmental-cleanup/aml

Various transnational global mining companies (Rio Tinto, et al) have extensive datasets on global resources and minesites, both operational, and past and potential future sites.