The thing to keep in mind is that when you are printing two pages per side double-sided you have four images per sheet and if they are then being center-stapled, the images will need to be out of order. So, I guess it is multiples of 4 rather than of 8.

For a 4-page booklet, you would have the first sheet be page 4 and page 1 on side one and page 2 and 3 on side two.

For an eight page booklet, you would have page 8 and 1 on side one, pages 2 and 7 on side two, pages 6 and 3 on side one, and 4 and 5 on side two of the second sheet. You can't easily get Word to flow this way.