Messages can be published as fast as needed, however there max throughput is going to be limited by network bandwidth, server/device CPU/RAM limitations, ISP, just to start.
On the subscribe side, you will have same limitations and you should consider how many messages a subscriber can consume based on the use case. If it is messages that a human needs to read then how many messages is too many? If it is data that is driving a realtime graph, then consider how many per second is enough to provide the necessary graphing experience.
For further guidance, please contact PubNub Support and we will assist with best practices to implement high message rates to satisfy your requirements.