Channels are "server-side" message filters. If a client isn't subscribed to a channel they won't get messages on those channels. This means it is advantageous to subscribe to lots of tightly scoped Channels rather than only subscribing to one or two channels where you send messages that not all clients will necessarily be interested in. If you only send messages that clients want you reduce the amount of data being sent to the client and also reduce your messages count!
In short designing your application with lots of Channels for specific subjects is better practice than subscribing to few very busy Channels.
There is no limit on the number of Channels that you can subscribe to: