Seminal fluid is alkaline, so it reduces the acidity sperm experiences in the female reproductive system.
So even with careful selection and breeding controls you still want a few hundred.
the more other things like skills matter the more people you need becasue you can't just select for genetics which brings you closer to 500 individuals.
And, perhaps most important, seminal fluid gives the sperm the energy (in the form of fructose) they need for their arduous swim.
According to Morgentaler, semen comes from three main structures in the male reproductive anatomy.