Learn Hardcore Russian: Сколько яблок ты купил?

In Сколько яблок ты купил? the word яблок is in the genitive plural. After the quantity word сколько (“how many”), the counted noun always takes the genitive case rather than the accusative. Many Russian nouns form the genitive plural with a zero ending, so яблок is the correct form for “apples.”

Why is the verb купил in the masculine singular form?

Russian past-tense verbs agree with the subject in gender and number. Here the implicit subject is ты, and if you’re asking a male friend, you use masculine singular купил. If you were asking a female friend, you would say Сколько яблок ты купила?

Can I omit ты in this question?

Yes. Russian is a pro-drop language, so the pronoun ты is optional when the verb ending makes the person clear. You can simply ask:
Сколько яблок купил?
Including ты adds emphasis or clarity but isn’t grammatically required.

How would I make this question more formal or address more than one person?

Replace ты with вы and adjust the verb accordingly. For formal or plural “you,” ask:
Сколько яблок вы купили?