One card will seem accurate, but don't buy more than one, because then you will never know which one is accurate.
It is not really that critical however, lots of things work very well, like a porcellian dish or a sheet of cheap white copy paper.
The card should be white, or light gray, but the 18% cards are too dark for white balance. The 18% varies too, not that WB cares about 18%. Everything neutral "works", even black can "work", but white is the best choice (to actually show the color cast present). The concept is named White Balance. Gray introduces pigment errors which have to be controlled, where white is a no pigment situation.
I find the cheap three-card sets to be the worst. But still far better than nothing.
About the only one that actually claims any accuracy specifications is the WhiBal card. It costs more, but they claim to actually control and verify the pigments. It is good, but the colors can come out a bit stark with it. It costs more to control the problem that the pigments introduced.
My own favorite is the 5x7 inch Porta Brace White Balance card, not likely in local stores, but only $5 at B&H. It is nothing fancy, just unpigmented white plastic (reminds me of PVC pipe), but a known neutral color, and it works very well. Washable, durable, cheap enough to cut some smaller (for shirt pocket, etc).
I don't use them to preset the Nikon camera. That seems too awkward, and takes too long, and screws up the next situation unless disabled (and it still depends on camera Auto WB, which I am trying to avoid in the first place). I use the the WB card to just place it in the same light in the first test shot, and then using the Adobe White Balance tool to click them, to remove any color cast there (in that known color). It is fast, Adobe raw allows correcting many images in the same one click (assuming all in in the same light situation).
The large mounting board is a white foam board from craft store - less good than anything here, but it is pretty old.
Not obvious here, but the one at right immediately above the womans face is an ordinary cheap white envelope. Ordinary cheap white copy paper to its left. Then two Porta Brace cards, bought a few years apart.
White craft paper background under the lens cap... Many things work well. It should be neutral, but it's not rocket science. I often rely on white things naturally found in the scene, T-shirts, church steeples, signs, etc. For anything serious (studio sessions, etc), I use the Porta Brace card.
