You can use an LOD expression for this. The calculation below counts the number of sessions per user:
If you use that as a Dimension and then you count the number of users, you get this:
The workbook is attached. Also, here are some common examples of LOD expressions: Top 15 Tableau LOD Expressions (Practical Examples)
Hope that helps!
SessionsPerUser.twbx 1.9 MB
That is exactly what I needed, Santiago. Thanks so much for the help!