What is the API rate limit for MailBluster, and can I update multiple leads using Developer API?

The default rate limit for MailBluster Developer API is 10 requests/second and 100 requests/minute.

And, you cannot update multiple leads using MailBluster Developer API.
Only single lead can be added at a time. Same goes for update and delete through the Developer API.

If you want to import more than one lead at a time, then you need to use the MailBluster main app.

Here is the API documentation for your assistance: https://app.mailbluster.com/api-doc.