All of them that I can tell apart. I even came up with "group names" for when I can't tell them apart. The ten neons in our "Hogwarts" tank are names The First Years. They tend to stay in a group and swim around looking confused.
I like themes, but those tend to break down after a few deaths, then I am tired of that theme and the replacement fish don't fit. I end up with something like a half Hogwarts / half Lord of the Rings tank, etc.