The total number of channels that you have for an account enables you to make the same amount of concurrent calls on the Account. Each call, either incoming our outgoing takes up 1 channel. However, if Call Forwarding is turned on, you will need a minimum of 2 available channels for a call to be connected. This is because as the incoming call comes in, 1 channel is used to handle the call, and when the call is forwarded, another channel is used to make the call. But once the call is connected, the first channel will be released and can be used for another inbound or outbound call.
You can add additional channels directly on the portal under Account Configuration and any updates are updated in real time.