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 your Channels account. The number of channels subscriptions that each connection holds does not impact the connection count. 

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?