How does "waitlist" work?

I plan to take the Chemistry Subject Test on the 5th of December. I’m waiting for my SAT 2 scores from november before registering because I might have to sit for Physics again. I’m confused about how waitlist status works? So I’m supposed to pay $100 (almost) for the ticket and then try my luck at the centre? What happens if I don’t get a spot? How often does one get a spot?

If you don’t get a spot, then you don’t take the test. How often one gets a spot depends on the testing center. I assume most people here are so paranoid and conscientious about SATs that they would never ever risk registering to the wait list.