When adding something like bourbon (or other whiskeys) to the secondary, how much do you add? I realize this will differ based on recipes, tastes, and styles, but what is a good range? How much is so minimal that you won't taste it, and how much is so much that it will overpower the grains and significantly increase the ABV of the final product?
I would say don't add it to secondary at all. Add it a little at a time at bottling, mixing and adding to taste.
Track the amount you use and if you make the beer in the future, you can just add that entire amount again.
That said, I have had good luck pitching a 12oz-16oz of bourbon into 5 gallons and getting decent flavor. Depends really on the flavor profile of the base beer.