Committers are users who author VCS (version control system) changes for projects that are built by TeamCity. Once a user authors 10 changes in a VCS, they will occupy one committer slot. Committer slots are released after 30 days of inactivity.
In general, each developer in a project will require a committer slot in TeamCity Cloud. Regular web users are unlimited in TeamCity Cloud. QA, managers, or other team members who do not author changes will not take up committer slots. They can freely view build results, modify build configurations, and perform other actions in the TeamCity user interface.