App Store Connect API randomly returns error "An unexpected error occurred on the server side" or "The request timed out"

I use the App Store Connect API to run many parallel requests to update different parts of a single app. I am randomly getting errors such as

An unexpected error occurred on the server side.

or

The request timed out.

Usually when these errors happen, I can simply run the unsuccessful requests one or two more times and then they succeed.

Is there an explanation for this? Is this possibly caused by too many parallel requests? What is the maximum suggested number of parallel requests?