san,
konnichiwa!
I'm sorry for the late reply!!
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.
Hope this helps!
Natsuko (奈津子),
Team JapanesePod101.com