How many collections can i add into a database

I know that a document have something like 16 MB limit, if i want to create for every forum topic of my website a new collection and into that collection i save all posting of a topic, then i would have maybe after some time a lot of collections in the database, will that be a good practise or how would you recommend to slove this, is there a limmit how many collections you can create or is it for any other reasons not recommend to code like that?

You might find the following interesting.

thank you i will check that, so it sounds like it is not good idea to create a collections for something like every new forum topic.