The funny thing about the PJO cabin system is that everyone's always all 'oh the twelve' this and 'the twelve' that but that's absolutely not even remotely accurate. To start, right off the bat it's thirteen, not twelve, because they don't count Hades. But not really because before Percy, there were no big three kids, so we're down to ten active cabins already but it's actually eight because Artemis and Hera don't make demigods.

And of those eight, Mr. D is stuck at camp (thus not really making new demigods all that often) and his only two kids don't even sleep in a cabin, they sleep in the Big House with him.

So, pre-Percy, there are seven active cabins at Camp Half-Blood:

Glee club, the Jocks, the Nerds, the Geeks, the Farmers, the 'Sketchy Kids' and the Popular Kids.

Or, in other words, the Apollo, Ares, Athena, Hephaestus, Demeter, Hermes (and the unclaimed kids) and Aphrodite cabins.

What's cool is that you can already see the cabin dynamics in the show. For example, the Athena cabin allies with the Hermes cabin for the numbers. The Hermes kids plus all the unclaimed kids? It's the biggest cabin in the camp by far. It's a battle strategy. Luke and Annabeth's close relationship is just the cherry on top for Annabeth. It'll be really cool to see how the show develops the differences in the cabins during the series.