In the CosmosDB documentation, Microsoft hints at a throughput limit on a single partition, but does not specify the limit. We is the limit?. Here is the relevant documentation: https://learn.microsoft.com/en-us/azure/cosmos-db/partition-data
And the relevant quote:
Behind the scenes, Azure Cosmos DB provisions partitions needed to serve T requests/s. If T is higher than the maximum throughput per partition t, then Azure Cosmos DB provisions N = T/t partitions.
Doesn't explicitly answer your question, but the reason this value is not explicitly mentioned is because it will be changed (increased) as the Azure Cosmos DB team changes hardware, or rolls out hardware upgrades. The intent is to show that there is always a limit per partition (machine), and that partition keys will be distributed across these partitions.
You can discover the current value by saturating the writes for a single partition key at maximum throughput.