
I don't think there is a set rule. I feed my lot part barf, part complete food. They have chicken wings once or twice a week and on those days I substitute one meal for the chicken wings, usually the breakfast. My border collies get 2 wings each and the borzois 4 each - so probably 3 or 4 for your boxer, these are the asda smartprice chicken wings which are actually quite big and meaty :)
Mine also get a whole egg once a week or so...I give them the whole egg, including shell in the garden. I love watching them eating it - youngest collie drops hers straight away on the slabs and wolfs it down, the others carry them off and lick them for a bit before piercing them :D
Another treat they love that is readily available is oxtail, and of course raw knuckle bones.