Billing depends on whether you are connected to the Matter Enterprise Grid or not.
Who can use this: Workspace Owners and Admins on any paid plan.
For everything on this topic, see the Plans and billing collection. Related articles: How does adding or removing channel members affect billing? and How is the billing user count calculated?
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. See Enterprise Grid for Slack or Enterprise Grid for Microsoft Teams.
To learn more about the Enterprise Grid, contact us here or schedule a time.