36, 72, 144....
any number divisable by 9 evenly.
the idea being uniformity and balance for alignment.
It can be tricky to do a 144, but I have seen it done.
Good Luck !
I have a five hole but would need to make some new rubber spacers to use it. If someone is building a bike with a 40 spoke wheel, this sproket is ideal. I might be persuaded to part with it if someone needed one...Kelly