I see that campaigns can be created via the API, and within the customers section, enter the phone number and the variable values. Do I do this through the API? Is there a limit to the number of customers available through the API? Is it possible to make the call with 2,000 or more customers, or would it have to be done manually with a CSV?

Yes, you can add phone numbers and variable values via the API, use multiple API calls in batches or upload a CSV through the dashboard to avoid size limits..

Yes, but exactly what would be the exact limit? With the campaigns API, I believe it can only be created once with the total number of customers. In theory, these campaigns would manage the issue of the concurrency limit for outgoing calls, so as not to have to create batch calls through the API that may not be sent due to the concurrency limit.

You can include 2,000+ customers in a single campaign via the API, as there’s no strict limit so vapi handles queuing based on your concurrency cap..