How do I know whether I qualify for this bonus interest rate?

Hey Anon, it's not stated anywhere on their platform whether there are 20,000 customers. Either way, just sign up for the account and deposit at least $500 and check the next day to see the amount of interest earned (not credited). From there you'll know if you're still within the first 20k customers entitled to the 2.5% interest p.a, since opening an account is free of charge.

You have to try first? maybe they will update you on it...