Known issue. Currently multipart uploads can only be uploaded at 2 parts per upload ID concurrently. We have a fix pending that should fix that bottleneck within the next month or so (maybe sooner). The change will show up on https://developers.cloudflare.com/r2/platform/changelog/
For now there are typically settings you can configure in whatever client you're using to lower the concurrency for uploads.
To answer my question the answer is no, this is about concurrent requests, so it's 2 at once but it will do upto 500 for each as many times as it needs to, to upload a 20GB file.
For now there are typically settings you can configure in whatever client you're using to lower the concurrency for uploads.