In the documentation for:
https://api.qualtrics.com/9f1dc7785bd26-get-participants-post
I do not see any limitation on numbers of calls, but if I run 100 calls in parallel, then the response after approx 65 of them are: 429 - Too Many Requests
 

Is there a limit on this type of call or am I hitting another limit?

I also raised a request for office hours 3 weeks ago, but have not heard anything back?