I've always used a single card. That's all. Well, that's not really true - back when I started out in digital, cards were small and wouldn't fit that many photos, so I carried two or so, and a portable hard drive to back up the card to. Of course I didn't use the latter much, and as soon as memory went down in cost, I just bought a bigger card. That's what I use now. None of what I do is 'mission critical'. If it would be, I might carry one or two spares. And/or use a camera that records images onto two cards simultaneously. As it is, in the past 15+ years or so of shooting digital onto a single card, I've lost maybe one or two frames to data corruption. The other tends of thousands of frames are happily clogging up my network drive.