It scales very far. We've conducted benchmarks lately that showed that a single deepstream on an AWS t2.medium instance can comfortably handle 160.000~200.000 messages a second and a cluster of six ~ 4 Billion an hour. Please find the detailed results here

https://deepstream.io/info/performance/four-billion-messages...

For your chat though you need to consider whether your messages will be persistent and mutable (changeable after they've been posted) or are just one-off messages. In the first case using a list (https://deepstream.io/tutorials/core/datasync-lists/) with references to records (https://deepstream.io/tutorials/core/datasync-records/) is the way to go, in the second case pub/sub (https://deepstream.io/tutorials/core/pubsub-events/) might be better