Billing depends if you are connected to the Matter Enterprise Grid or not.
If not, each channel is billed as a separate account. If team members are in multiple Matter channels, they will be counted separately for each channel.
The reason is that each channel has its own experience and value - its own workspace settings, customizations, etc.
However, if connected, Enterprise Grid billing is the unique aggregate number of channel users across all Matter-connected channels.
To learn more about the Enterprise Grid, contact us here or schedule a time.