We have 5 action cameras in the house so there are a few micro SD cards kicking about. I'd love to say we're organised and have them all neatly lined up in an old Ferrero Rocher box we specifically set aside for storing USB drives and memory cards, but we haven't. Oh, sure, we have the box, and it's got the USB sticks and a couple of card readers in it, but if a card isn't in a camera then it could be anywhere. Except in the box.
I'd guess though there's at least a quarter of a terabyte's worth of cards not in cameras which may be on shelves, in cupboards, or parts unknown at any given moment. Generally we pull files from our cameras direct every day or two, and don't tend to switch cards unless the Drift's have done their "little thing" again.So we only realise we're a card short when it turns up in with the teabags or wherever. I do always have a spare in my bag though, in a SD adapter tucked into a little zipped pocket I'm guessing was designed for an MP3 player or something. Because my Ghost-S is in another of its 'Low Speed Card' phases.