As I only 'upgraded' to Windows 8 then on my desktop I had the download and key for it only for 64-bit. My laptop was 32-bit and again the download and key is tied to 32-bit. I don't know if it's the same if you buy the full retail Win8. With Win 7 the full retail came with two discs, a 64 and 32-bit versions and you could use the same key to install either or.
Surface Pro (2017) vs Surface Pro 4