In our web app, we have a large amount of data associated with each user. So we are thinking of allotting a separate database to each user. Hence, this will require a large no. of databases.
So, my question is -- is there a limit on no. of databases, like there is a 24,000 limit on collections in a single database.
If you use one database per user and there's no flow of data between databases, then there's no limit, since you can always put a certain fat client's database on its own server.
If you were talking about sharded cluster, then again I think you're fine. The more databases you have, the more memory config servers will use. When config server outgrows its machine, then you know you have quite a few databases.