how much vanilla extract do I use when substituting seeds from 2 vanilla beans?
It's one teaspoon per bean for a milder flavor, or 2 teaspoons per bean for a stronger vanilla flavor. If you're trying to sub for 2 beans, use 2-4 teaspoons of vanilla extract. I'd recommend starting with 2 teaspoons, then tasting it and adding more if need be.
I would go more like two tablespoons but only good quality vanilla extract. What is the volume of the recipe? You could also gauge it by that. Something like 1/2 tsp per cup of flour, very generally speaking.
If you want to have real vanilla flavor but don't want to buy expensive vanilla beans or go through the effort to de-seed them, you can buy vanilla paste, which is just the seeds and "tar" from the insides of them. It is a great shortcut.