> And who knows how many of them happened during the past millennia.

We at least have a significantly large list of what we know [1] - that's part of the purpose of core drilling, the ash deposits worldwide can be linked together to estimate where ash traveled to. Also, craters and their surrounding can be drilled into to determine eruption events.

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