Why is SQL Server Express creating twenty users on my system?
Do you have installed the R- and Machine Learning services? According to the documentation below those are generated to run external scripts of those services in an isolated environment. So I would say they are normal if you install those components.

Those are part of the Advanced Services Variant of the SQL Express Edition.

From the documentation: Modify the user account pool for SQL Server machine learning


  By default, the user account pool contains 20 user accounts. In most cases, 20 is more than adequate to support machine learning tasks, but you can change the number of accounts.
  
  
  In a default instance, the individual accounts are named MSSQLSERVER01 through MSSQLSERVER20.
  For a named instance, the individual accounts are named after the instance name: for example, MyInstanceName01 through MyInstanceName20.