The amount of fluid (not sperm) you shoot is limited by the capacity of your seminal vesicles, a right/left pair "storage tank" glands that are between the prostate and the bladder. Mammals like pigs that shoot a lot of cum have huge vesicles compared to humans.

You can probably take various supplements and stay hydrated to get to your genetically programmed maximum capacity, and maybe increase slowly over time, but it's hard to imagine them getting to enough capacity that you'd shoot a whole cup full.