Horizontally scaling a MongoDB database for a flask API and providing user based access control

Question

I'm trying to create a Database structure for a flask REST API in MongoDB using MongoEngine. The requirement is to make a horizontally scaled Database cluster - one Db per user. The user should be able to access only his/her own database. Every time a user is to be added to the system that user should only be able to access his assigned database. What is the more efficient way to do so? What are the best practices for doing this?