The number of photos that see the light of day / number of clicks.
As I said, not so simple. I take many photos for the sole purpose of checking the histogram and doing ETTR (and I should really get off my arse and do proper UniWB tricks). These shutter releases are never intended to be photos but are simply a tool to measure exposure properly).
Actual failure rate of straight delete due to technical errors of photos that aren't used to check histograms are about 0-10%, whether anyone other than me or my family sees them is different to the numbers seen by clients or sold commercially.
So I guess technically my keeper rate is 90%, photos I actually am extremely proud of are about 1 in 4,000, but in taking those 4000 photos i will have generate hundred that are perfectly respectable and are making me money.