Vanilla Porter - Vanilla bean question

I'm making a Vanilla Porter using Midwest's Porter extract kit. My plan is to soak some cut and scraped vanilla beans in a couple ounces of bourbon and put that in the secondary.

I'm wondering if anyone has done this. I'm unsure how many beans to use. I've read anywhere from one to three. I'm going for an obvious, but not overpowering vanilla flavor. I just want the vanilla flavor to blend nicely with the coffee and chocolate notes of the porter. (Assuming it has coffee and chocolate notes. This is my first time making the Midwest Porter)

Any advice on doing this, or even just general secondary flavoring advice is welcome.