.net drive connect to multiple db's with diferent user credentials

I have mongo cluster with multiple databases. I have created a single MongoClient and can access to different databases based on application logic using this single instance (saw the explanation in this topic .net driver & multiple database). But when iI tried to apply RBAC control on databases access (e.g. each database has it’s own set of users with roles for access) i don’t see a way to pass the user credentials into GetDatabase function. it’s is a part of the MongoClient. Do I need to create a MongoClient for each Database? if so what are the performance cost of having Many MongoClients and i assume this will result in many connection pools . Is there any limitation on how many can be created?