I have added 1 oz vanilla extract to 6 gallons and after 1year tasted really good, but that is my taste. I would not go any higher. I just added a little bit at a time, tasted to get the amount that the Mrs and I liked.
I use a Fermotemp wrap. Holds temps to what you set it at, easy to use, I tape it on to carboy using either painter's tape or electric tape. Cost around $35 .