@Baaridun Nasr- When you trigger a Teams bot hosted on Azure Bot Service via a POST request from an external web app, the message count on a premium channel is calculated based on the number of messages sent to users.
If you are sending a message to multiple users, each message sent to a user count as a separate message. So, if you send a message to N users, it will count as N direct line messages on the premium channel.
To see the total premium messages sent and received by the bot, you can use Azure Bot Service's analytics and monitoring tools. Application Insights, which is integrated with Azure Bot Service, provides detailed telemetry and analytics, including message counts. You can access this information through the Azure portal by navigating to the Application Insights resource associated with your bot