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Very long exp darks will need more sub exposures compared to short exposures for a different reason. Assuming that cosmic ray hits appear at a reasonably constant but random rate, the longer the total exposure is, the more likelihood that they will affect your dark exposures. They are removed essentially when you combine your darks but are not part of the dark current and can leave residual noise. The more exposures you have, the better they are removed.
Shorter exposures will have less hits and as 10x 30 sec frames is less total exposure than 10x 20min exposures you can get away with less subframes for the same noise in the final master dark.
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