How much liquid vanilla extract should I use in a five gallon batch of porter?
You're really the onlyu one who can decide that. We don't know what your beer tastes like or the flavor and strength of the extract. The best thing to do would be add to taste after fermentation. Pour 4 small ,measured samples of the beer. Add a different, measured amount of extract to each. Taste, determine which you like best, then scale that amount of extract to you batch size.