Sata disks! Pair only?

I’m puttinh a box for a customer of mine, who happens to be a friend, and I recomended a SATA disk due to price. I thought that the hole pair of 2 thing wasn’t mandatory. I thought that there was a way to link one of those disks alone without a need for a second!

Do SATA disks only work in pairs?

That sounds terribly stupid if u need to buy 2 to use as one…
If this is all there is… This is one seriously useless technology…
There is a way to link one and only one SATA disk as a boot disk? Right?

Please get back to me on this! The box is a present for a little girl and it’s supposed to be done at the end of the week! Tops!