1.
how many
Used to ask about quantity, especially for items counted with the general counter つ.
2.
how old
Used to ask about someone's age, particularly for children or in casual contexts.
幾つ is one of the question words for quantity and is often written in hiragana. For asking age, it's commonly used for children or in casual situations. For adults, 何歳ですか or お幾つですか (polite) is more appropriate. The word corresponds to the counter つ (hitotsu, futatsu, etc.).