I have 5 right now in a 20 tall.
Would have more but i have plans for other fish so i'm holding off on adding to my initial group until i decide on the final number of other fish. Most people recommend a 20 long though, saying that since theyre bottom dwellers that they'd appreciate more length. Mine however have never used the length of the tank to swim back and forth like my old dojo loaches did. They mostly just do loops up the right hand side, circling the corner filter i have there. Loop loop loop all day long. These are my 3 black kuhlis, who are always out visible and active. I've never not seen any of them, they are always out, being the stars of the circus. I wouldnt normally have gotten only 3 but that's all the store had at the time. The lack of a large group hasnt hindered these guys in any way, they have never been the least bit shy, since the minute they went in the tank theyve been bold little clowns.
So I originally got 5 striped ones too, to try to create a decent sized mixed group at least.- so far from them: one died in the first few days. One has hidden under the driftwood constantly since i got him. A second one just finally reappeared about a week ago. One of those two this week has recently started venturing out with the black ones - not looping, but hanging out in their "landing area" so they swoop by him. The other two striped ones i havent seen since day one. If they died, i havent found a body. Not in the filters. I've done pretty thorough vacuuming several times but nobody has been stirred up. So hopefully thpse two are just chilling in the sand somewhere. But i've kind of stopped counting them. But if you are counting those absentees, i have 7 in a 20 tall. The way my crowd behaves, i would easily have physical room for twice that. I'm only waiting on filling out theur group because of potential bioload of other fish.
Although the striped ones are finally starting to come out of their shell a little bit, its taken almost 2 months.
Because of this, and from my own experiences of having had black kuhlis in past years, and now this batch, so far i prefer the black ones to the striped ones for activity and entertainment.