Well, the board supports up to 128GB of ram. If you're looking to occupy all of the slots then you can stick to kits with 2x32GB. Keep in mind if you decide to overclock the maximum ram capacity on this board, it'll be up to a certain speed. I doubt you would be able to overclock it over 4000MHz. I don't know anyone at the moment that has a high ram frequency with 128GB installed. But that day will come eventually with new BIOS released.
Anyhow, if you decide to play with 64GB, it's better to occupy all dimm channel's, which is in this case 4x16. There's something about it. More stability, more efficient & better overclocking ability. Currently, I don't see why you wouldn't overclock 64GB of ram into the extreme. Most users out there attain extreme values with 16 or 32GB, however 64GB is definitely achievable.