Before answering the question, let us see 万 | 萬 evolution history first: ( image get from http://www.vividict.com)
|Learn one Chinese Character a day - 万 | 萬|
Looking at Oracle Script of traditional version 萬, does it not look like a baby stretches out its hands to ask for something? Bronze Script added a handto mean to hold the baby.
|Baby held in arm|
It is now quite obvious that Chinese ancestors considered the next milestone of a person is "to have offspring". So and were used to mean next number after "千" as "Ten Thousand".
Evolved and been simplified further from Seal Script to Clerical Script , it is very close to what we used now for traditional version of "Ten Thousand" - 萬.
As for simplified version 万, it suddenly appeared in Clerical Script. Chinese Linguists guess that it was related to Buddhism symbol 卐.
万 (Simplified Version) | 萬 (Traditional Version) means "Ten Thousand".
Some example numbers:
123456 - 一十二万三千四百五十六。
Now it is time to enjoy a Chinese Calligraphy with "万 | 萬" inside.