I am planning a recipe with Tonka Beans. At what point should I add them and approx. how many? I know that vanilla is usually added after primary. Should I do the same with these or can I add them already during the boil?
Tonka beans contain Coumarin which the FDA in the US has banned the use of these beans because of hepatotoxicity (liver damage). That being said adding vanilla beans to the boil is a bad idea as the flavor compounds will evaporate off. Putting them in primary or secondary is the best plan. Have added vanilla beans to secondary an that gives great flavor to beer. The amount to add is really the kicker.
Another possible approach that is commonly used would be to soak the tonka beans in vodka and add the tonka bean flavored vodka to the finished beer when you are done. The benefit of this approach is you can test adding different amounts of the tonka bean flavored vodka to beer in glasses and see what amount gives the effect you wanted. Then simply scale it up and add the calculated amount of tonka bean flavored vodka to the beer.
Good luck and let us know how it ends up tasting. My guess is that small doses of tonka beans should be fine. Acetaminophen also has liver toxicity but it is still available over the counter as an analgesic. ;-)