Most of the time, adding yeast at bottling time is not necessary since enough healthy yeast remain in suspension to consume the priming sugar. But in cases where the existing yeast might not be sufficient, I was looking for a reference to for the quantity of healthy yeast to pitch at bottling time.
The balance seems to be between excess yeast sediment and speed of carbonation. If there is no hurry, it seems like a very, very small bit of fresh yeast would be sufficient. But to complete carbonation faster, more yeast would probably be required.
I am aware that Sierra Nevada bottle conditions their beer and they add yeast at bottling time:
Bottle conditioning is a time honored method for naturally carbonating beer and traditional champagne. We dose back a small amount of fermentable sugar and yeast into the bottle. The fermentation creates the finished carbonation and flavors unique to our beer. https://www.sierranevada.com/faq/beer
Despite adding yeast, I don't recall any dregs in those bottles. So they must add very few yeast cells, but I don't see the specifics.
Are there any sources or references for the minimum effective quantity of fresh yeast pitched at bottling time?