Notice the difference in meaning between che and quale. The question Quali film hai visto? asks "Which films have you seen?" (For example, Il Postino, Ciao Professore! or La Vita ? Bella.) The question Che film hai visto? asks "What types of films have you seen?" (For example, comedy, romance, or thriller.)

The interrogative adjective quanto (how many, how much) agrees in number and gender with the noun it modifies and has four forms (see table below).

FOUR FORMS OF QUANTO? (HOW MUCH?/HOW MANY?
 
masculine/singular
 quanto
 Quanto tempo fa? (How long ago?)
 
masculine/plural
 quanti
 Quanti anni hai? (How old are you?)
 
feminine/singular
 quanta
 Quanta farina c’è? (How much flour is there?)