oh boy do you end up with a lot of them.
You just need more practice with the Japan Coin Simulator ;)
https://wendal.itch.io/japanese-money-simulator
Theoretically the most coins you should ever have is 15:
4x 1 yen
1x 5 yen
4x 10 yen
1x 50 yen
4x 100 yen
1x 500 yen