A little while ago, scientists did a study to see if schooling fish can count. They would show an individual fish two schools of the same type of fish, plus something threatening like a big fish behind glass. The fish chose two over one, three over two, and four over three. Beyond that, they didn't appear to register a difference.
In other words, some freshwater schooling fish can count to four. Somehow I doubt it matters too much whether you have eleven or twelve chromis. I know odd groups are suggested for freshwater fish, but I think it's mostly because odd groups look more natural.
I have to ask, though... Why are so many people trying this? I mean, I know a few people have succeeded and it's awesome-looking, but there have to be at least a few thousand chromis dead because people tried to get schools of them.