I guess different tanks. Different hobbyists. Different opinion. I have N. vibex in all 9 of my tanks except 1. They don't clean algae, this is very true, but they are carrion eaters. The vibex don't munch on your sandbed critters. There are alot of look alikes that are not beneficial for a clean up crew, but that's where research comes in before buying. Recognize what you are looking for by sight.... not the label on the glass. I consider them important because they eat the dropped food and dead critters that can be fuel for an algae outbreak. The other snails may eat the algae, but the vibex helps clean up the potential fuel. If there is frozen mysis that hit the substrate there are two candidates to eat it up.... the BWs and the nasses. The vibex will beat them to the punch everytime... and if not, they'll just take it. I'd rather feed a nass than encourage an unpopular BW outbreak.
How many I would recommend would depend on potential available food and the square inches of sandbed real estate. But when I'm not sandbed heavy, I just put 1 in there to pick up the dropped groceries.
I love snails and they are not just a part of my CUC, but get as much consideration as my fish...I brought them here... they deserve at least that much. I like a nice variety, for example in my 50g tank I have 3 types of ceriths, nassarius, nerites, dwarf planaxis, periwinkles, trochus, astraea, collonistas, fighting conch... I know there's more but I can't think of them at the moment. There is enough film algae behind the rockwork where I can't see to keep them happy if the front gets cleaned too often. If I have a turbo or something in my tank that is running out of food...I supplement them until more develops. Now I don't have a "whole load" of snails.... just a small variety of each... or a single specimen depending on what it is.