Too many SQL Server users
You're running SQL Server 2016 with R Services enabled. These accounts are created as part of installation, and are used to run R processes when called from SQL.
From the MSDN documentation:
Step 3: Enable Implied Authentication for Launchpad Accounts
During setup, 20 new Windows user accounts are created for the purpose of running tasks under the security token of the SQL Server Trusted Launchpad service. When a user sends an R script from an external client, SQL Server will activate an available worker account, map it to the identity of the calling user, and run the R script on behalf of the user. This is a new service of the database engine that supports secure execution of external scripts, called implied authentication.
You can view these accounts in the Windows user group, SQLRUserGroup. If you need to run R scripts from a remote data science client and are using Windows authentication, these worker accounts must be given permission to log into the SQL Server instance on your behalf.