Six otocinclus or more should be kept together. Otocinclus are social creatures and feel safer and more comfortable living in a large group.
A wild otocinclus school can consist of thousands of fish. For that reason, you should keep at least six otocinclus in your tank. With that said, the bigger the group, the better. If you have the space, you should aim to keep 10-20 of them in your tank.
Just make sure your otocinclus get plenty of space to swim around. Fortunately, these fish are rather small and don’t need much space. As a general rule, an inch of fish requires one gallon of swimming space, so a 10-gallon tank is sufficient for six otocinclus.