According to the docs:
Functions are limited to 30 concurrent invocations - meaning that if you have more than 30 Functions being invoked at the same time, you will begin to see new Function invocations return with a 429 status code. To keep below the threshold optimize Functions to return as fast as possible - avoid artificial timeouts and use asynchronous calls to external systems rather than waiting on a large number of API calls to complete.
-- https://www.twilio.com/docs/runtime/functions/faq
How can I obtain the execution time metrics of my Twilio functions? In order to ensure my functions stay under the 30 concurrent invocations limit, I'll need to compute the number of concurrent invocations, based on execution time and number of requests. I need to know the execution times (and ideally the number of invocations, but I can get that elsewhere). Does Twilio provide any metrics for Twilio Functions?
Twilio developer evangelist here.
I don't believe that information is available either in the response headers or via the API right now.
If you are concerned about breaching those limits, I would recommend you raise this with your account executive at Twilio. If you don't have an account executive, that sort of usage sounds as though you should have a relationship, so get in touch with sales or failing that support.