There are people around here who have 10.000+ books in their library. I don't think there's a upper limit to your library size apart from practical (memory size) matters.
You can actually add any field you'd like to to your library. The feature is called "custom columns", see "preferences->custom columns" or
the manual.
If by classification system you're thinking about for example the Dewey system, there's actually been some discussion as to whether that should be implemented as a column. The consensus, though, seems to be that at least for the time being, people wanting to use such a system should implement it themselves by adding an appropriate custom column.