Go as if you're actually trying to book a stateroom (pick a specific cruise, then go to select a stateroom). It'll show you a map of available staterooms for that category and location. It's a bit of a pain because you can only look at one stateroom type and ship location at a time, so it's a lot of clicking back and forth. But you can generally see how many staterooms are open. (Keep in mind though, as I noted above,
DCL doesn't always list all available staterooms on the website so it might not be super accurate. But if will give you a general idea.)