Not the parent commenter but not always. 9 pool just means you build a spawning pool at your main, for instance. This worker-prefix building build-order naming system also breaks down once people start referencing builds like 2 rax academy, 3 hatch muta, etc.
Right, "9 pool" means build a spawning pool when you have 9 workers. So "12 courtyard" means build an expansion when you have 12 workers.
I think strictly "9 pool" means you build the pool when you have 9 supply. However, before you build a spawning pool, the only thing you can build that consumes supply is workers.