Thsee are what I think are correct:
Je veux le café = I want the coffee
Je veux le café = I want coffee
Je veux du café = I want some coffee
Ils veulent des cafés = They want coffees
Ils veulent du café = They want some coffee
ILS veulent le café = They want coffee
Ils veulent de café = They want a coffee.
I think these are all correct grammar, depending on the situation.
Please correct me if I'm wrong.