Always at least two; one fixed and one folder. Sometimes I'll add a multitool to that, but I don't really count that as a "knife." I count that as a "multitool."
I carry two at all times. I was taught the use of the knife as an "anti-grappling device," and in the words of the instructor, "you never know which side you're going to fall down on."
Strong-side knife is used for utility duties as well, but is kept sharp.
2 minimum...Victorinox multi-tool on my belt and a Victorinox mini in my pocket (the scissors & tweezers are really useful). Occasionally a Kershaw assist in my front pocket.