I think this post should have gone to the oddballs sections...anyways,
in a 15gal tank you can have 3. As for sexing, it's next to impossible sexing juvenile DPs. In short,
the female is more rounder than the male,
the male has a brownish line at its ventral side running from head to tail (more likely the throat area to tail),
and most significantly, the lines behind the males' eyes known as wrinkles. This is the best method for sexing DPs as the shape can be rather difficult to judge and the line at the ventral side of the male may not be clear under stressful conditions.