Mattermost Cloud is offered as a pay-as-you-go subscription service.
You will be billed for registered users in your workspace. A registered user is a user who has an account in a workspace and does not show as Inactive in System Console > User Management > Users. Registered users will count towards your monthly invoice each day as long as they are not deactivated in the billing period.
Mattermost Cloud workspaces can be used free forever with Cloud Starter. You can upgrade to a monthly per-user subscription, with Mattermost Professional and Mattermost Enterprise.
Please see the Cloud subscriptions documentation for more detailed information on the Cloud billing process.