Another practical limit would likely be the transaction size. All of the work in a transaction is logged and the limit to the size of that log is 16MB.
Consider that if there are many Indexes on a single Collection, then the transaction log will contain everything from all of those indexes – you might quickly hit the transaction size limit. If you have many Collections, and just a few Indexes on each, you are less likely to reach the transaction limit for just writing a Document.
Note that it is possible to hit this limit when creating just one document with a single, very complex Index. See this previous topic: Transaction exceeded limit of 16MB with 56kb file
Here is another discussion about the transaction log size: Transaction exceeded limit: got 24179504, limit 16777216