Do I have to buy the whole slab/sheet?

Slabs are almost always sold intact. Buying random slabs would be analogous to buying fabric remnants. Like a tailor, your fabricator buys the raw material and bases his price on the complete fabrication and installation of your job. The number of slabs and the amount of resulting waste is determined by the size and configuration of your countertop. The fabricator will lay out your job in a way that minimizes waste and maximizes the natural beauty of the material you selected.