So,
after some surprisingly frustrating research, it turns out that the average volume of tomcat interject measures 0.01-0.8ml.
There are 4546.09ml to a single Imperial gallon.
As a result, this means that the number of cats, or the number of ejaculations, required to provide 1 gallon of cat semen is anywhere from 5682 to 454,609.
That's right, if you chose to drink the semen, it is possible that you're drinking the combined semen of almost half a million cats.