There are two sets of numbers in Korea, the native Korean and the Sino-Korean Numbers. Sino-Korean numbers are borrowed from Chinese. Generally speaking, for expression involving dates, money, foreign loanwords, minutes, seconds and counting beyond 99 Sino-Korean numbers are used otherwise its the native Korean numbers.
You might be wondering why some numbers are skipped in both numeral systems, this is because there are certain patterns to achieve the numbers in between.
In Native Korean, to express let say 11 it’s 열하나 (yolhana) so the formula is 10(열)+1(하나). This is the reason why after the number 10 only numbers in multiples of 10 are provided up to 90. So you have to memorize at least 18 numbers in Native Korean.
While in Sino-Korean, to express let say 32 it’s 삼십이 (sam-ship-i) so the formula is 3(삼) X 10 (십) + 2 (이). There are few numbers to memorize in Sino-Korean, with just 13 numbers to you can go up to a million already.
See related posts on how to deal with numbers. Check the archive.