(that's Lexar new 256GB Professional 3333x CFast™ 2.0 card) and the answer can change to "1"
That's a whole lot of eggs that will fit in that one basket. I think I recall my momma saying something about that…
If you accidentally format the card, you can recover images. Or the card starts acting funny. Just don't shoot until you do.
So you never make a mistake and your cards are flawless, sure 1 card is enough. If you are human or your equipment has an occasional glitch, like the rest of us, then a few extra cards will help.
I switched from 16GB to 64GB and that made me nervous. One card takes from 2500 to 3000 RAW images and if you wait until it´s full before you swap, a faulty card or any other fxxx up may be quite disastrous. In both my 1DX and 5DIII I duplicate all images on the two available cards, but I either swap cards or download every day.
But I remember when I was thrilled to get my first 8GB CF card, so I suppose 256GB is a candidate for being standard when we go to +45MP images.
1DXII, 5DIV, 5DSR, 8-15/4L, 11-24/4L, 16-35/2.8L III, 24-70/28L II, 70-200/2.8L IS II, 100-400/4.5-5.6L IS II, 200-400/4L IS 1.4x, 17/3.5L TS-E, 24/4L TS-E, 35/1.4L II, 100/2.8L IS Macro, 300/2.8L IS II, 600/4L IS II, Zeiss: 15/2.8, 35/1.4, 55/1.4 Otus, 85/1.4 Otus, Profoto B1