Is Zakat paid once or repeatedly?

If someone has $20,000 left in saving account in year 2022 (after they pay zakat for it), and in year 2023 they are able to save another $5000, should the zakat be given as 2.5% of the $5000 or 2.5% of the $25000? I noticed a similar question but with no answer. I would appreciate references and sources for the answer.