I like to collect knives with different steels, and only keep a max of 3 knives in the same steel. For example, I have 3 in S30V, 1 in CPM M4, two in D2, two in 1095, one Damascus, 3 AUS 8, 2 154CM, 1 in Sandvik 14c28n, 1 ZDP-189 and 2 VG-10.
I also like different handle materials as well. I've got leather, zytel, paracord, frn, aluminum, titanium and a few others.