Answered my own question… loads and loads of tests available! I was too afraid to set up an account in case I had to follow through with booking. Nevertheless, I am now booked for several weeks out and it gives this procrastinator a deadline to work toward!
I did mine a few years ago, now, but that's what I needed, too. Book it to give me a hard deadline, and then I had no choice but to prepare. Good luck!
Unsolicited tip: I had the app and was "studying" solely by setting up mock test situations, and reading thoroughly all the ones I would get wrong. Before long, I was consistently getting 24 right on every go. But a few days before my test date, I decided to do an online practice test for the sake of completeness... and I failed. Needless to say this put me in a state of panic and bewilderment. How could the official app have let me down? I wanted to just chalk it up to an outdated website, but I couldn't. Passing was too important, so I took time to read (the section of) the book I missed the most in: the early history stuff... in my opinion, the app doesn't cover this area adequately. After just a cursery skimming through this section of the book, along with my work with the app, I easily passed the test on the day. All of that to say: don't rely solely on the app, like I nearly did. There were a couple of questions on the early history stuff on my actual test, and at least one that I wouldn't have known without the skim-reading of that part of the book.
But
do use the app! It's really good, other than that oversight.