Back in the film days the number I heard was if you got 1 or 2 printable photos from a roll of 36 you were doing good.
I think the number of printable photos depends on the standard. My standard has been is it worth the money to actually print the image.
AND now is it worth my time to process.
I took photos at a couple of gymnastic meets at NCSU back in the 90's. The gym lighting was HORRIBLE. I used 800 speed Portra film pushed two stops to 3200.
It was UGLY but what was needed to get the shutter speed. I spend over $300 on film and development and did not get one photo that was worth a dang. 
If I do macro photography I would guess my success rate is 50-75% depending on the subject and if I am playing with aperture for DOF the success rate might go lower just because I take more shots.
Successful/printable landscapes or animal images are back toward the 1/36 ratio. Or worse.
Images of kids basketball in a decently lit gym I am getting a printable rate of 32% to 47%. The first two weekends I was taking about 400 images and getting around 130ish images worth processing and printing. My workflow is to assign a "3" to a decent image that is sharp, has a full face, ball and the hoop if possible. If that image has the extra thang of movement, emotion, whatever it gets a 4.
I am not finished with last weekends processing since I took 800 images.
Some of the kids stared playing games so there was more photo opportunity and I was trying out some things. The first two weekends each generated 400 images of which 130ish were 3 and 4 rated photos. About 40 images were 4 rated. So if we say that only a 4 is worthy of printing then I was at 10% rate.
For the last couple of vacations I have taken about 1200 images in a week. About 800 I have kept. Printed in some way, mugs, puzzles, photos, magnets, a core of 30-40. Used in a slide show 300-400.
"What percentage of shots are print worthy?"
It depends. 

Later,
Dan