how can pornstars drink alot of cum (sperm) ?

How do you know they do? Porn is often fabricated/exaggerated. It is also dangerous.

How much is a lot?

Why the question? Do you want to and can't?

They do porn scenes after 3-4 months so they produce more , some eat zinc medicines to produce sperm.And some foods like eggs , fruits etc also help to produce sperm.

Go ask your mother , :D  , she will tell you the secret , by the way I should say pain chod teri maaaKiki cus mai lun kutti maa ka bacha randi ka bhi

I think zinc would have negative impact on sperm count. Fruits and veggies, along with lots of water. Ginseng and alike supplements would also increase sperm count.
But the question was "how can pornstars drink cum?"
My guess would be either they truly love the taste and texture of a load of sperm inside their mouth. Or they do it for the money and to satisfy the porn industry's request for sloppy, sperm messy mouths, and cum thirsty appetites.
Sperm is actually very healthy to consume. It is also a very powerful anti depressant for woman. Most importantly, any woman who hungrily makes love to my penis, with her mouth. And then she hungrily gulps down my entire load. She is a forever keeper.