Upload requests limit

You should not make multiple calls to files_upload itself at the same time for the same user, as that will result in lock contention. Please read the Data Ingress Guide for detailed information on how to upload many files:

https://www.dropbox.com/developers/reference/data-ingress-guide

So, is your doc line "We recommend uploading many files in parallel to increase throughput." incorrect?

I don't understand. If I have 100 files less than 150 MB each what is the correct way to upload?

No, it's not incorrect. It's referring to using using the upload session batch functionality to do so. (Specifically, that line is from the documentation for /2/files/upload_session/finish_batch.) Please refer to the Data Ingress Guide for information on using that. You can use that if you have to upload many files, even if they are each smaller than 150 MB.