This answer is late however the other answers seem a bit…weak in terms of reliability and factual information so I will attempt to remedy that a little.
But for some reason mongodb set limit 24000 for the number of namespaces in the database,
That is merely the default setting. Yes, there is a default setting.
It does say on the limits page that 24000 is the limit ( http://docs.mongodb.org/manual/reference/limits/#Number%20of%20Namespaces ), as though there is no way to expand that but there is.
However there is a maximum limit on how big a namespace file can be ( http://docs.mongodb.org/manual/reference/limits/#Size%20of%20Namespace%20File ) which is 2GB. That gives you roughly 3 million namespaces to play with in most cases which is quite impressive and I am unsure if many people will hit that limit quickly.
You can modify the default value to go higher than 16MB by using the nssize parameter either within the configuration ( http://docs.mongodb.org/manual/reference/configuration-options/#nssize ) or at runtime by manipulating the command used to run MongoDB ( http://docs.mongodb.org/manual/reference/mongod/#cmdoption-mongod–nssize ).
There is no real reason for why MongoDB implements 16MB by default for its nssize as far as I know, I have never heard about the motto of “not bother the user with every single detail” so I don’t buy that one.
I think, in my opinion, the main reason why MongoDB hides this is because even though, as the documentation states:
Distinct collections are very important for high-throughput batch processing.
Using multiple collections as a means to scale vertically rather than horizontally through a cluster, as MongoDB is designed to, is considered (quite often) bad practice for large scale websites; as such 12K collections is normally considered something that people will never, and should never, ascertain.