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Many sources mention 온 (100) as the highest Native Korean number; I even heard a native Korean tell me that 온 isn't actually used, and 아흔아홉(99) is the largest actually usable number.
Is 99 - or 100 - really the limit of what can be expressed?
If so, what is the historical reason for this? It would seem to make the number system impractically limited, and yet it has clearly developed and survived.
Old Korean at least had 온(100) and 즈믄(1000), and maybe more, though I don't know if higher numerals were recorded elsewhere. Your native Korean friend is correct in that 온 is no longer used to mean 100: its only meaning in modern Korean is "whole", as in "온 천지가 눈으로 가득하다 (Everything is covered in snow)".
As for why, my uneducated guess is that Koreans were accustomed to Chinese numerals for a very long time, so some native numerals just lost the competition and died out. Similar things happened to 뫼(->산 mountain), 가람(->강 river), 범(->호랑이 tiger), 저자(->시장 market), and many others.
For fun, also consider that English no longer has a native Germanic word for "second", even though they can say "nine thousand, nine hundred and ninety-ninth" in pure Germanic words!
Below are all the numbers expressible in Korean(a lot).
You can see that the highest number is 10^68, which consists of 68 zeros.
Most of there words have disappeared because of lack of usage.
However, some still exist in everyday talk, only that they don't represent actual numbers anymore(ex-'찰나의 순간' stands for a very short period of time, and you can see that '찰나' originally stood for 10^-18, a very small number.
Also, like many other Korean words, there are actually two kinds of expressions for numbers in Korean: the'순우리말'system and the'한자어'system.
The answer above explained about the former system, and this answer consists of the latter.
청정(淸淨) 10^-21
허공(虛空) 10^-20
육덕(六德) 10^-19
찰나(刹那) 10^-18
탄지(彈指) 10^-17
순식(瞬息) 10^-16
수유(須臾) 10^-15
준순(逡巡) 10^-14
모호(模湖) 10^-13
막(漠) 10^-12
묘(渺) 10^-11
애(埃) 10^-10
진(塵) 10^-9
사(沙) 10^-8
섬(纖) 10^-7
미(微) 10^-6
홀(忽) 10^-5
사(絲) 10^-4 = 0.0001
모(毛) 10^-3 = 0.001
리(厘) 10^-2 = 0.01
분(分) 10^-1 = 0.1
일(一) 10^0 = 1
십(十) 10^1 = 10
백(百) 10^2 = 100
천(千) 10^3 = 1000
만(萬) 10^4 = 10,000
억(億) 10^8 = 100,000,000
조(兆) 10^12
경(京) 10^16
해(垓) 10^20
시(枾) 10^24
양(穰) 10^28
구(溝) 10^32
간(澗) 10^36
정(正) 10^40
재(載) 10^44
극(極) 10^48
항하사(恒河沙) 10^52
아승지(阿僧祗) 10^56
나유타(那由他) 10^60
불가사의(不可思議) 10^64
무량대수(無量大數)10^68
