Based on the level of respect or formality, you can say "How many are you?" in Korean in 3 ways:

• Informal: neoheui-deul-eun myeot myeong iya? (너희들은 몇 명 이야?), neoheui-deul-eun myeot myeong ini? (너희들은 몇 명 이니?) - neoheui-deul-eun can be omitted.
• Formal: dangsin-deul-eun myeot myeong iyeyo? (당신들은 몇 명 이예요?), dangsin-deul-eun myeot myeong ipnigga? (당신들은 몇 명 입니까?) - dangsin-deul-eun can be omitted.
• Very Formal: dangsin-deul-ggeseo-neun myeot bun isipnigga? (당신들 께서는 몇 분 이십니까?) - dangsin-deul-ggeseo-neun can be omitted.

- how many: myeot myeong (몇 명), myeot bun (몇 분)
- you (subjective plural): neohui-deul-eun (너희들은), dangsin-deul-eun (당신들은), dansing-deul-ggeseo-neun (당신들께서는)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.