I also have a question about the number 21 in NATIVE Japanese numbers. For instance I know "ni-juu ichi" is 21 in the sino-japanese counting system but I'm trying to count in the native counting system beginning with 1=hitotsu.
I know 10=to and for numbers after that you add "amari" so that 11=to amari hitotsu. My question would be does the system continue to keep the same form going forward after 20=hatachi? Where 21 would be written hatachi amari hitotsu. And if you can also numbers like 121 where 100=Momo.
To be honest, the Japanese counting only goes up to 10 these days. Only a very minority (Japanese) would even know the word 'amari'. We do use 'hatachi' for the age of 20, but in the contemporary Japanese, we use only up to 10.