TWO minimum on duty (uniformed patrol LEO).
I may sound like a hypocrite-I start getting "the weird outs" when I meet some folks that carry more than 3 or 4 on their person for general EDC. When I was SCUBA diving-I had 3 (leg, BC, gauge console) and when doing anything outdoor I usually have 3 or 4 (fixed blade belt knife, light folder in right pocket, small lightweight fixed blade on pack strap, SAK in pack).
On duty I carry a plain edge folder in my right from pocket and a serrated edge folder in my left front pocket. For about a year or more that was PE Endura and SE Matriarch 2. A Cold Steel Black Talon 2 has bumped the Endura for now...at least on a trial basis.
Off duty I carry the BT2 in right front pocket but always have one more knife on me somewhere, although I usually don't bother with a serrated edge off duty. I ALWAYS wear a vest (khaki, canvas, fleece, or down) off duty (to conceal off-duty gun, mags, badge) so I usually drop a Delica in the right pocket of that garment.
TWO MAIN REASONS-
1. Use for most "public" displays and/or to loan to someone, so as not to scare the poop out of them or make them think I'm psycho by the "holy crap!" from-the-pocket deployment of the BT2!
2. Easier to get to seated than the BT2 in the pants pocket.
Blue Titanium/Jigged Bone: Endura Damascus, Delica Damascus (finally!)
Foliage G10: Endura FFG, Delica FFG
OD Green G10: Worker Sprint
Black FRN: Endura FFG, Delica FFG, Dragonfly FFG, Matriarch 2, Lil Matriarch, Centofante 3, Salt 1 PE
Grey FRN: Endura FFG, Delica FFG
Purple FRN: Endura FFG, Delica FFG
Green FRN: Delica FFG
Denim Blue FRN: Lil Matriarch PE (Thanks McL!)