Quoted from https://www.italki.com/question/372129
It depends on the counting noun.
if the counting noun is Sino-Korean, the numbers read as Sino-Korean.
if the counting noun is Native-Korean, the numbers read as native-Korean.
There are some exceptions.
건, 개, 명, 시간, 근, 시 are used with native-Korean number
살 is native, 54(살) read as native.
세 is Sino-Korean, 54(세) reads as Sino-Korean.
When reading as Native number, the number which is above 100 read as Sino and the number which is below 100 read as native.
6번 => 육번(for number), 여섯번(for times)
6분 => 육분(for minutes), 여섯분(for people)
6개 => 여섯개
6 points => 육점 or 여섯점
600 points => 육백점
6 people => 여섯명
600 people => 육백명
6x => 여섯배
3 languages => 언어 세개, 세개의 언어