Hi Jack,
I understand you are looking for information on rate limiting for general API usage. I believe you will find this guide (or here in case you are using the older Adwords API) provides quite useful information related to your question, in particular this section (here for Adwords) on concurrent threads. Depending on your use case, Batch Processing (here for Adwords) may be a useful means of lowering the number of requests sent as well.
Regards,
Laura, Google Ads API Team