מָאתַיִם עִזִּים צְרִיכוֹת עֶשְׂרִים תְּיָשִׁים, וְכֵן כֻּלָּם, הַזְּכָרִים כְּדֵי צֹרֶךְ הַנְּקֵבוֹת;
... לְפִי שֶׁהֵם פְּנוּיִים מִמְּלָאכָה דַּרְכָּם לְהַרְבּוֹת תַּשְׁמִישׁ וּלְעַבֵּר עֶשֶׂר נְקֵבוֹת, וּבְהֵמָה מִשֶּׁנִּתְעַבְּרָה אֵינָהּ מְקַבֶּלֶת זָכָר, וּפָרִים שֶׁעוֹסְקִין בִּמְלָאכָה, לֹא מָסַר לְזָכָר אֶלָּא אַרְבַּע נְקֵבוֹת, וְלַחֲמוֹר שֶׁהוֹלֵךְ בְּדֶרֶךְ רְחוֹקָה שְׁתֵּי נְקֵבוֹת לְזָכָר, וְלַגְּמַלִּים שֶׁהוֹלְכִים דֶּרֶךְ יוֹתֵר רְחוֹקָה נְקֵבָה אַחַת לְזָכָר:
Two hundred she-goats have need of twenty he-goats, and so too in the case of all the various species, the males were in number according to the need of the females. ... ten she-goats were given to each he-goat, and so [too] to each ram; [as] since they are free from work, their way is to be frequently involved in sexual relations and to impregnate ten females - and once an animal becomes pregnant, it does not accept a male. And [concerning] the bulls that engage in work, it only gave four females to the male; and to the donkey who goes on long journeys, [it gave] two females to the male, and to the camels that go on [even] longer journeys, [it gave] one female to the male. (Gen 32.15)
IMHO, if only one mating is needed to impregnate a female, one male is sufficient for any number of females, given enough time, but nobody mentions the עונה requirement for the animals (like once every x days). Also, I don't know of any empirical support that working animals (bulls) have lesser mating patterns than non-working (rams).
What is Rashi's logic here?