This is an opinion piece. I have an opinionated opinion.
Illustrators and cosplayers alike are widely varied in their approach to the number of diamonds on Harley's costume. Since there is little consistency in the illustrations themselves, this makes it difficult for cosplayers to select the "right" number. There doesn't really seem to BE a "right" number.
Placement is a related issue, at least as far as illustrators are concerned. Some (not all) illustrators get really over-enthusiastic and start putting the diamonds all over the place.
But the placement of Harley's diamonds is really very specific and not at all random. They're meant to evoke a sense of the old patterned Arlecchino costume, not reproduce it. Too many diamonds makes the costume too busy from a stylistic sense for a dainty female character in a streamlined design, and they're placed to be symmetrical and therefore flattering. The majority of illustrators faithfully adhere to the proper placement and cosplay