This is the 287th most frequent Korean word.
명이
A counter for people in Korean.
In this example, '명이' describes the number of individuals that comprised the completing team.
그 프로젝트를 완수한 팀은 세 명이었다.
The team that completed the project consisted of three members.
Here, '명이' serves to describe the number of people related to the noun '사람' who attended the lecture.
강의에 참석한 사람이 여섯 명이었어요.
The number of people who attended the lecture was six.
In this sentence, '명이' is used as the subject marker, connected to the number and counter '명' to indicate the quantity of students present in the classroom.