Messages can be published as fast as needed, however maximum throughput will be limited by factors like network bandwidth, server/device CPU/RAM limitations, ISP, to name a few examples. Our recommendation is to publish 10-15 messages/sec per channel.
On the subscribe side, you will see the same limitations so 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.
See, also: Soft & Hard Limitations
In case of any questions, contact us at firstname.lastname@example.org.