I keep the ones with good traits and keep the amount relating to the characters I have that can make use of them. For example, I only have a single stam NB, which I rarely play and have no intention of ever making another one, so I'd only keep 1 of each monster set that would prove useful if I were to decide to use my stam NB. I don't see the point in keep two monster sets of the same sets if i have no characters that will make use of them even if they're dupes in good traits.
I have one mule now that carries only monster sets. I only ever keep the best trait I have for each armor weight. Some sets I collect multiple times (Veli and Kragh 'cause I have 2 Stamina dds that use them).