At a guess, 25-30, which is rather more than a household of two needs - especially since we hardly ever have enough visitors to need more than half a dozen at a time.
We seem to keep acquiring new ones too, although we keep a cap on numbers through judicious breakage - unfortunately, it always seems to be the mugs I actually like that get broken. Most recently, I knocked something off a shelf that landed on my lovely Ladybird Books 'The Story of Football' mug, putting a big crack through it that renders it useless. Pah! Not sure of the capacity of that mug but it was over a pint. I could get a replacement off ebay but it wouldn't be the same.
We also have some very nice Anta tartan stoneware mugs. I often feel like I ought to get rid of all the others and just keep those ones, but they're very expensive to replace when they get broken.
