For what to capture, check this discussion:
https://forum.inaturalist.org/t/photo-taking-recommendations-by-taxon/1962
Number of photos is up to the user. Probably doesn’t make sense to be overly redundant, but you never know when one particular angle might be just what an identifier needed for a positive ID. Personally I try to be pretty selective and only include my sharpest images of whatever features I captured.
iNaturalist resizes all photos uploaded (including from Flickr) to a maximum of 2048 pixels in the long dimension. So there is no need to worry about “over-resolution” photos, except to the extent they might slow down the uploading process while iNat takes the time to resize the big ones. But final storage size is controlled by iNat. And yes, iNat stores its own copies of Flickr photos that get uploaded.
Hope that covers what you were asking.