What kind of questions should you ask during requirement gathering for APIs?

I've been working as a dev for a few years and been starting to own projects more end to end from the requirements stage. There are obvious questions/SLA like what are the business needs and how fast the APIs must be, but I'm not sure what questions I should be asking that may save a lot of headaches in the future. In this case they need an Api which would call around 20 external vendor APIs to aggregate different analytics data into a single json response which would eventually be moved into snowflake. What should I be thinking about and questions should I be asking? we are using Java and any other tips on how to build this would be super appreciated, thanks!