Hangeul is the writing system used for the Korean language. It consists of 10 single vowels, known as "vokal tunggal" in Indonesian. These vowels are ㅏ (a), ㅓ (eo), ㅗ (o), ㅜ (u), ㅡ (eu), ㅣ (i), ㅐ (ae), ㅔ (e), ㅚ (oe), and ㅟ (wi). Therefore, the correct answer is 10.