For sigle walled CNT it depends on how the CNT is formed. In ideal CNTs the structure can be characterized by two numbers.
1) How many hexagons are there as you go around the circumference : n
2) Where do you end up once you go around by one turn as you go step by step from hexagon to hexagon [In the simplest case you end up in the hexagon where you started. But the structure may actually exhibit some helicity {such as a screw} as you go one turn around (doig n steps) you end up having moved a number of rows along the nanotube axis], the number of rows you have moved forward be m, say. Clearly, m