it's just a recommendation, not a must-do. I've 4 otos in my 30gal tank and they're doing well. I also have 3 in my gal CRS tank. otos are not schooling fish and they mind their own business, meaning that they're not very interactive with each other. so i wouldn't worry about it. But if you have the money, i would suggest you get more since they're good algae eaters and i can't praise enough of the job that they're doing in cleaning up both of my tanks.