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I find somewhere around 20 dark frames to be pretty sufficient even with my noisy-as-all-hell Canon300D.
If you are flat-fielding using an LCD or EL panel rather than the sky, these are so bright (and your flats so short in exposure) that dark flats are quite unnecessary. Hard to do though with large apertures, or remote sites. I do consider it very important to get a large number of flats using this method though (at differing rotational angles and side-side translations) because of imperfections or smudges on the panels.
hope this helps,
Cam
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