in my area they are pretty cheap, i think this is due to the fact that they are one of the smallest plecs, im not sure about numbers as ive not looking into the species, but you need to have bogwood in the tank for them to rasp on, thye get nutrition from it somehow, they arnet a very good eater of algae tho so if you want something to get rid of the algae look into bn's, or otto's, they also like fresh greens so give them cucumber and veg a few times a week!
Im not sure they shoal, no i im pretty sure they dont( someone could clear this up?), but there would be no need to get 6 unless you are crazy for them, they will be fine in lower numbers
price: around £4-5 each