Five to six otos can be kept in a 10-gallon tank. As a rule of thumb, one inch of fish requires at least one gallon of swimming space. Considering that otos are usually two inches long when fully mature, each oto will need approximately two gallons of space.
However, keep in mind that this calculation only applies to species tanks. If your 10-gallon tank is a community tank, you must also take other tank inhabitants into consideration.
That said, this arrangement isn’t recommended. Since otos are schooling fish, they feel most comfortable living in groups of at least five or six otos. So, a 10-gallon community tank with other fish living in it would be too small to accommodate a small school of otos.