Currently bucket sharding is applied at the global level. In S3, bucket sharding is dynamic and/or increased by the operator upon tenants request, eg. in anticipation of a spike in utilization. https://docs.aws.amazon.com/AmazonS3/latest/dev/request-rate-perf-considerations.html
I have closed this issue because it has been inactive for some time now. If you feel this still deserves attention feel free to reopen it.