〜つ
general counter (one, two, three things)
一つ, 二つ, 三つ … 九つ, 十
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Japanese doesn't count things with bare numbers; it adds a counter. The all-purpose one uses the old native numbers plus つ:
ひとつ, ふたつ, みっつ, よっつ, いつつ, むっつ, ななつ, やっつ, ここのつ, and とお for ten (no つ).
It works for most things that don't have a special counter, and it's the safe choice when ordering: コーヒーを二つください. Past ten, you switch to the ordinary numbers with 個 (じゅういっこ).
The counter usually goes after the thing and its particle, right before the verb: りんごを三つ買いました, "I bought three apples".
Examples
りんごを三つください。
Three apples, please.
部屋に椅子が四つあります。
There are four chairs in the room.
質問が一つあります。
I have one question.
コーヒーを二つお願いします。
Two coffees, please.
箱の中に卵が六つあります。
There are six eggs in the box.