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.

For example:

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?