The verb gustar refers to the direct object. The literal meaning of gusta is "it is pleasing" or "it pleases." Le is referring to the person who is pleased by the object. Therefore, gustar refers to the object and not the person. If the object is plural, then it will be gustan.
¡Hola, Cristina! Thank you for using the Jiskha Homework Help Forum. Yes, many have trouble with the verb "gustar" because they learn it as the verb "to like." Forget that meaning and change it to "to be pleasing..." That will explain why it is most often used in the 3rd person, singular and plural. That is NOT to say that you will never see the other forms; there is a popular song entitled "Me gustas tú" which means "You are pleasing to me" or "I like you!"
Here are examples for which you asked.
A Nicolas le gustan las ciencias políticas. That means "literally" "To Nicholas, to him are pleasing political sciences. It would, however, be translated into "Nicholas likes political sciences."
ONE thing is pleasing to someone: gusta
TWO or more things ARE pleasing: gustan
The Indirect-Object Pronoun says to whom:me (to me), te (to you), le (to him, her, you-formal-singular), nos (to us), os (to you-all familiar plural), les (to them = people or things = to you-all, formal and plural
Please feel free to ask any other questions you may have. Hopefully there isn't too much here to confuse you!