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It depends on the context of the word. "Die" is for feminine words, "Der" for masculine, and "Den" for neutral.
N. Azzerri has written:
'Definition of a continuous neutral electrolytic pickling (den) process entirely devoid of noxious industrial releases'
Yes, the noun 'den' is used as a collective noun for:
Den's, as in
"What a coincidence we both have Den's" or
"The bear's travel to different Den's each night".