The number of concurrent connections you can have is based on the maximum number of connections as described by your Channels plan: https://pusher.com/channels/pricing
Concurrent connections means the total number of simultaneous connections you can have open across you channels account.
1) User A connects to your Channels app - that is one concurrent connection
2) User B connects to your Channels app as well - that is two concurrent connections
3) User A disconnects from your Channels app - you now have one concurrent connection again.
4) User C connects to your Channels app - you now have two concurrent connections (user A is still disconnected and so is not counted: remember we are counting simultaneous connections).
Also see: How are connections counted?