Just for history
https://aws.amazon.com/blogs/database/scale-write-performance-on-amazon-documentdb-elastic-clusters
We configured five elastic clusters with 1, 2, 4, 8, and 16 shards, respectively, each with 32 vCPU shard capacity. The application was run with a batch size of 1,000 for 5 minutes while varying the concurrency on each elastic cluster.
We configured an instance-based Amazon DocumentDB cluster with a primary and a replica instance. We chose db.r6g.8xlarge instances with 32 vCPUs for the instance type and Amazon DocumentDB engine version 5.0.