I give them a new box every time they've built out and are using six of the eight frames. I don't use a queen excluder so the queen builds brood in whatever boxes she would like. In the spring build up, I often have brood in the bottom three boxes although they usually share the top box with a few honey frames on the sides.
Remember that an 8 frame box is 20% smaller than a 10 frame so it makes sense that the brood would need three boxes.
So I don't keep a rule in my head about how many boxes - instead, I just give them a new one every time they need it.
I typically overwinter in three medium boxes. But I didn't harvest from all of my hives last year and didn't consolidate a few of them - so they wintered in four or five frame boxes - here in Atlanta we barely have winter now that global warming is so prevalent, so it doesn't really cause a problem.
Linda T in Atlanta