According to this document, using a buck on one doe per day will maximize sperm concentration. The text below is from
http://www.fao.org/docrep/t1690E/t1690e05.htm
Sperm production.
The volume of semen ejaculated is about 0.3 to 0.6 ml. Concentration is evaluated at 150 to 500 × 106 spermatozoa per ml, but both volume and concentration are liable to vary. False mountings, one or two minutes before copulation, increase the concentration of the ejaculate. In two successive servicings the first acts as a preparation for the second, which is less voluminous but more concentrated. During subsequent matings the volume of the ejaculate decreases, while concentration increases between the first and the second ejaculate and then diminishes. The total number of spermatozoa per ejaculate follows the same trend.
Maximum spermatozoa production is obtained by using the buck regularly once a day. If the buck is used regularly twice a day, each ejaculate has only one half the concentration of spermatozoa. On the other hand, if bucks service several times a day, one day a week, the three or four ejaculates may be concentrated enough to effect fertilization. Further ejaculates contain very few spermatozoa and cannot effect fertilization often enough to be worth while. Daily spermatozoa production is roughly 150 to 300 million, independent of the rate of ejaculation. The maximum epididymis reserve is only one to two billion spermatozoa, only partially mobilizable for repeated ejaculations.