No, otocinclus cannot live alone. You should only keep otocinclus in groups. A small group of five or six otocinclus would feel right at home living in a 10-gallon tank.
With that said, wild otocinclus usually live in groups that have thousands of fish, so the more otocinclus you keep in your tank, the better. If you have the space, aim to keep 10-20 otocinclus in your tank.
Otocinclus are social fish, and living with other otos makes them feel safe and comfortable. If they are kept alone, they become bored and distressed. An otocinclus should only live alone when it’s sick and needs to be temporarily isolated from its peers.