스무 -- twenty.

This is a contraction of the word 스물. It is used when counting people or things.

• 스무 살 twenty years old
• 남자 스무 명 있어요. There are twenty men.
• 볼펜 스무 개 주세요. Can I have twenty pens please?

[edit] See also

• Counting in Korean
• List of Counting Words
• 스물
• 이십