This usually happens if a committer has different VCS usernames across multiple VCS services (e.g. GitHub, GitLab, Bitbucket), or across multiple environments (e.g. Git on a home laptop and office PC), and these extra usernames have not been configured in TeamCity, or the committer has no associated user account in TeamCity.
When TeamCity detects a commit, it tries to link it to an existing TeamCity user based on the configured VCS usernames and username patterns. If a commit can't be automatically matched to an existing TeamCity user, then such a commit is linked to an "unknown" committer.
To avoid "unknown" committers consuming your committer slots, we recommend registering all committers as TeamCity users and configuring the VCS usernames of each user, as described in our documentation.