Any channel can handle an unlimited number of messages published to it, and PubNub documents no maximum number of subscribers per channel.
The 100-message limit some readers have in mind is not a channel property. It is the in-memory subscribe buffer, which is scoped to a subscribe connection and shared across every channel that connection is multiplexed onto. It exists so a client that briefly disconnects can catch up; it is not storage. For durable retention, enable Message Persistence, which retains messages for your keyset's configured retention period.