how many rows can one partion key hold?

[wateray]

Hi all,
My team is using Cassandra as our database. We have one question as below.
As we know, the row with the some partition key will be stored in the some node.
But how many rows can one partition key hold? What is it depend on? The node's 
volume or partition data size or partition rows size(the number of rows)?
 When one partition's data is  extreme large, the write/read will slow?
Can anyone show me some exist usecases.
 thanks!