Twelve. The answer is twelve.
For whatever reason, we live in a world that can't decide on standardizing microwaves, TV remotes, hotdog-bun to hotdog-pack ratios, how to turn on the shower, how to take the bus, the best image format for the web and a host of other things but someone, somewhere along the way decided every shower curtain should have 12 rings, and by god, the shower curtain companies agreed and it's been smooth sailing ever since.
Don't take my word for it — see for yourself.
These are the matters at the forefront of my mind as I slowly move things into a new apartment (And, in keeping with yesterday's promise, a much lighter topic to dwell upon.).