"Je veux moins de soupe."
Traduction :I want less soup.
En anglais, on ne peut utilser of que s'il y a un article (the soup) ou un adjectif possessive (your soup, his soup etc) ou un adjective demonstratif (this soup/that soup)
Si non, on n'utilise pas of.
I have more books BUT I have more of his books
I would like more chicken BUT I would like more of the chicken (that you cooked sous-entendu)