That question is not as easy as it sounds. You need at least two additional beyond a standard 5.1 for height channels. But if you have four, it will make use of those. And if you have eight, it will make use of those. Of course, you need amplification for each of those channels, as well. But assuming you have the amps, one of the aspects on Atmos, is that it knows how to take advantage of all the channels you provide. From my perspective, for home theater, I would go with four height channels.
