How many books are too many books?

I am tempted to say that you can't have too many books, but that is patently not true, unless you are the British Library, or the Library of Congress or some other copyright library. I don't know how many my husband and I have, but it must run into quite a few thousand. Every time I try to estimate it, my head begins to swim. Let's just say we have nine walls of books and that two of those are long walls, one in a corridor and one along the wall of an attic. We are both academics, and over the years have amassed collections of books on our subjects: architecture for my husband, art history and literature for me. I started off with an English degree and still have all my text books, including an Anglo-Saxon primer. That's before we get on to contemporary fiction of almost every kind, particularly crime novels.
      I know really that most of the academic books I own won't be opened again - I am not planning to write another book on the Arthurian legends - but it's hard to get rid of them. I feel about books what my mother felt about clothes. She grew up during rationing when new clothes weren't easy to come by and loved to trawl the charity shops for bargains. Books were the same for me: desirable commodities that had to be carefully budgeted for. I haven't got over their relative cheapness these days and go on snapping them up in charity shops and second-hand bookshops. Truth to tell, it goes against the grain to leave a bookshop without a book.
       Recently we've run out of shelf space again and my husband is talking about making some more. But I have been thinking that maybe it's not more shelves that we need, but fewer books: heretical thought!  Ebooks have helped a bit. Some of the books that would be otherwise on the shelves are stored on my ereader or in cyberspace. But if I really, really like something, I want a hard copy, so they still accumulate. Perhaps the answer is to institute a 'one in, one out' policy. Or maybe to chose one every day to go to the Oxfam shop or to give to a friend. My dear friend Carola used to be ruthless about this and have periodic culls. Maybe I should do the same.
     I'd love to know how other people manage.