Best practise for handling library granularity

I was wondering if some users could share theire opinion of what makes sense how to organize libraries. Better have a lot of very small libraries or better have few and large once?
I’d like to backup a complete partition of 300Gb, about a million of files. Would it be complete nonsense to put it all in one single library?
It would have the advantage that every new file / folder is automatically part of this library.
The other extreme would be to have about 100 to 200 libraries, what would call for a lot of maintenance.

What are your experiences here?