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According to this there would be 250,000 stars within a 250 light year radius from Earth: https://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/LocalGroup/B...

So if all of those will explode eventually, that's a lot of supernovas (if the Earth is still around for all of those...)

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Not all of those explode eventually, rather it's a small amount. Only the most massive stars explode into a supernova. The majority of stars are smaller, and will have different lifetimes. For example, the sun, an unusually bright star (brighter than about 95% of stars) will eventually turn into a red giant, slough off material then collapse to a white dwarf. It's estimated that the majority of stars are brown dwarf (very small, some not too-too much larger than Jupiter), and less than 1% of stars are massive enough to go supernova. The problem is that only the big stars go super nova, and the big stars also have the short lifetimes because they're burning fuel so quickly.

White dwarfs can also explode, if they are in a binary system. It's a different kind of explosion (more akin to a thermonuclear bomb) but it also produces iron.

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