The original question is a good one, especially in a wide open sense (not just the expected 4-13 on this species).
Note that there is a monstrose form of the species whose primary attribute is its confusion about ribs, or lack of clearly defined ribs, if you prefer to look at it that way.
Either way, it's weird and cool.
The number of ribs on any given cactus sometimes goes into pseudo-Latin code, like with this cactus from northern Mexico that you may see described as "quadricostatum" (4 ribs), and indeed it was once its own variety of the species.
http://www.llifle.com/Encyclop...
"tricostatum" is another name that circulates in cultivation; these names seem to mostly exist in order for cactus growers to charge more money for plants that differ from the usual 5 ribs (the expected range is 3-8).
There is a fair amount of historical chatter about the number of ribs on cacti often being in the Fibonacci set (1,1,2,3,5,8,13,21,34, etc.) and you will see plenty of evidence of that all around you. I don't know why that happens, but it's interesting to count and check. The rib count is sometimes used to help distinguish varieties or subspecies of certain cacti.
