…since you became sexually active?
For the numerator, use whatever act you count toward your “number.” I’m not looking for an argument over whether a drunken BJ in a bathroom counts as a partner or not. Personally, I’m just counting intercourse.
For the denominator, use your current age minus the age that you lost your “virginity.” Again, your virginity is what you make of it. Personally, I’ve been active for 18 years, nine of which were in an exclusive relationship.