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. If you have Call Forwarding 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.
You can add additional channels directly on the portal under Account Configuration and any updates are updated in real time.
You can refer to this article for more information on