Granite comes in 2cm and 3cm thickness. The choice between 2cm and 3cm comes down to availability and personal preference.
2cm granite will have a plywood decking for added strength and a laminate edge to provide that thicker look. This actually results in a 4cm thickness countertop.
3cm granite will not require plywood decking or laminate edge. This thickness is already visually appealing and is sufficient enough for the countertop.