I have a project with 2 applications ( books and reader ).
Books application has a table with 4 milions of rows with this fields:
book_title = models.Ch
I'm not familiar with Django, but I have a general understanding of DB.
When you have large databases, it's pretty normal to index your database. That way, retrieving data, should be pretty quick.
When it comes to associate a book with a reader, you should create another table, that links reader to books.
It's not a bad idea to divide the books into subjects. But I'm not sure what you mean by having 20 applications.