It appears that there are many that download ANYTHING and EVERTHING just to put as much as they can on their readers.
I bet that there are a bunch of teenage boys who read nothing but manga. But with a couple of thousand harlequin romances mixed in to their collection downloaded from the dark net.
I prefer to keep between 100 -130 books on my reader, and I'm not even close to using the full capacity of the reader. I do have a more ebooks than that. But I use Calibre to manage what is on my reader, what I want to add to the reader, and what books have been read.
So if you don't have a compulsion to load as many books as possible, I wouldn't worry about more memory now. Give it some time. You can always add more memory later when you find that you REALLY do need 25,000 books on your reader.