Each brand of cat food has different nutritional content, so the recommended feeding amount will vary greatly. For example, my cat eats 60 grams of royal food a day, 80 grams a day if happy, and only 51 grams a day if competitive.
No matter what brand of cat food, the recommended feeding amount will be indicated on the package. You can reduce the standard feeding amount, or simply buy weight-loss cat food. Royal, Snow Mountain, Amus, Yilu, Mercy, and Hills all have weight-loss cat food.
If you buy Royal, the packaging will tell you how much you should feed a thin, standard or fat cat respectively