You should get at least three kuhli loaches. Although they’re not schooling fish, kuhli loaches feel more comfortable with some companions. They’re also very shy and if kept alone, they might end up hiding all of the time. Being in groups of at least three will make them feel safe enough to come out of hiding and explore the tank.
With that said, you should also take your tank size into consideration when determining how many kuhli loaches to get. A single kuhli loach needs at least 15 gallons of space, and you need to add at least 5 gallons of space for each additional kuhli.