Learn Italian: L'orchestra che suona stasera è famosa. - The orchestra that plays tonight is famous.

In Italian, collective nouns that refer to a group of people, like l'orchestra, la gente, or la famiglia, are strictly singular in grammar. You must use singular verbs like suona and è because the grammatical subject is one single entity, regardless of how many people are inside it.

Why does the adjective famosa end in -a instead of a plural ending?

Just like the verbs, any adjective describing a collective noun must agree with the grammar of the noun, not the number of people in the group. Since l'orchestra is a feminine singular noun, the adjective must be the feminine singular famosa.

Could I use gioca instead of suona for playing in this sentence?

No, because Italian has two different verbs for playing. You must use suonare for playing music, an instrument, or ringing a bell. Giocare is strictly used for playing sports, playing a game, or playing like a child.

Why is the relative pronoun che used here instead of cui?

Che is used when the relative pronoun is the subject or direct object of the phrase. In this sentence, that is the subject doing the playing. You only use cui when there is a preposition in front of it, like di cui (of which) or con cui (with whom).