Learn Russian: Пять тысяч зрителей купят билеты на концерт. - Five thousand spectators will buy tickets to the concert.

In Russian, numbers ending in 5 through 0 require the following word to be in the Genitive plural. Because тысяча (thousand) acts like a feminine noun, the number пять (five) forces it into its Genitive plural form, which is тысяч.

Why is зрителей also in the Genitive plural?

The word тысяча acts as a container noun (like a "group" or a "dozen"). Whenever you use thousands or millions to count something, the item being counted is always in the Genitive plural, regardless of how many thousands there are. So, the spectators become зрителей.

Why do we use купят instead of покупают for "will buy"?

Купят is the future tense of the perfective verb купить. We use the perfective aspect here because the sentence focuses on a one-time completed result: the spectators will successfully purchase the tickets. Покупают is imperfective and would mean they are currently buying or regularly buy tickets.

Why use the preposition на for "to the concert" instead of в?

In Russian, public events, shows, and performances always take the preposition на (to/at) instead of в (in/into). Because the tickets are directed toward this event (showing destination), концерт is in the Accusative case, which for inanimate masculine nouns looks exactly like the basic dictionary form.