A 10-gallon tank could theoretically accommodate around 3-4 Otocinclus, considering the thumb rule of one inch of fish per gallon. Otos are small, typically growing only to 1.5-2 inches, so their size permits keeping a small group even in modestly sized tanks.
However, Otocinclus are active and love to explore, and more significantly, they are social creatures. It’s crucial not to overcrowd them since this can lead to stress, potential health issues, and territorial disputes. If you plan on adding other species to the tank, then the number of Otos should be adjusted accordingly. In any case, always prioritize the welfare of your pets over any rule of thumb.