There is no limit on the number of sheets a user can create. There is no "auto-clean" as well. You can delete sheets createad by any user using the QMC, but pay attention, you can delete important sheets created by the users,
My recommendation is guide and nurture the users on best practices for self-service BI. Create new sheets only if needed. Show them how em when to remove obsolete sheets. Use clear descriptions and names for each created sheet so the userr can find a previous analysis more easy. Share with them the responsibility to keep the environment organized. It´s common for users start to maintain several copies of a sheet to track a version control of each change.
If the idea is to create an auto-clean up I recommend to create a separate app exclusively for sandbox, and on regular basis you can delete it from the stream and publish it again. When you do this, all objects created by users (Sheets, bookmarks, stories) are deleted too.
These are a few sugestions. Maybe other have some interesting ideas to share as well.