I haven't checked lately how many books I have on my kindle or I should say kindles. I mainly use my Oasis now so there would be more than say on the Voyage, which is the 2nd most used kindle. I didn't put all the books on at once of course. I put a few 100 on when I got my Oasis and by now it probably has close to 1000. I do not keep "read" books on the device. I delete those after reading and rating and filing to GR.
I have no issues at all with speed or anything on my kindle. Doesn't seem to matter how many or few items I have on it. Now that was a different story with the K1. I had a SD card on that one and 1000's of books and I remember it taking 8 minutes to search. That is not a joke, it actually took 8 minutes.
With the newer kindles though I have no issues. My K3 didn't have any issues either, nor my basic/button.
I don't think I have every single book I own on the device, not yet anyway. But I have many. I use kindle collections extensively so I never have any issued finding what to read. I have 3 levels of reading next type collections and a total of 47 collections. None of that seems to slow down my device.
Search is very quick also, but most often I go to collections as I don't know what to search for. When I just try to find a next book that is.
I guess I didn't need to put them on, but I want to, so there.

It has plenty of space for books, which is all I read on it.
I use calibre only for backup, not for pulling any books. I buy my books from amazon only so always pull them there. New purchases go direct to my Oasis so that is why the number keeps growing on the device.
I just like having them on there. So no matter what is going on, where I am at, I can always access most of my books. Who knows where my reading mood takes me.
I guess I just don't think about those things much. It is what it is. I backup my stuff to the cloud, in my case Amazon as its the cheapest. I would have used dropbox, but the free account is not enough for my book library. And I don't want to have a service where I have to bug strangers to sign up. I just want to pay for what I use, but I can't afford dropbox plan. No clue why they don't have a smaller one. So Amazon it is. I back up my calibre to a hard drive and to a stick drive thingie.
Otherwise I just sit back with my kindle, browse my lovely books and read.