Similar to Bed Count. One's Head count is the number of times you've gone down on someone, performed oral sex. Many people believe that oral sex doesn't count as a full notch on your belt, so Head Count is adjusted as Fractional Sex.
Kim: "Well, add one to my head count. I just went down on David in the back seat of his car. I guess I'm up to 15 now."
Krista: "Well, that's fractional sex, so it only counts as 10, especially if you didn't swallow."
Kim: "I swallow every time."
Krista: "Okay, 12.5 then."