Japanese counts things with counters that depend on what is being counted: 〜つ for general objects, 〜個 for small items, 〜人 for people, 〜枚 for flat things, 〜本 for long ones, 〜匹 for small animals, 〜冊 for books and 〜杯 for cups and glasses.
The hard part is the sound changes — いっぽん, さんびき, ろっぴゃく — so this drill asks for the full reading, and the ones you keep getting wrong come back until they stick.
Every card has a temperature. Get it right and its chance of coming back halves; miss it and it doubles. K2 picks the next card at random, weighted by that chance, so a session keeps returning to whatever you are getting wrong and lets what you know fade into the background.
When the cards you are working on have cooled, a new batch joins them, so you always study at the edge of what you know. There are no review dates and no daily queue to fall behind on. Every drill is free and works without an account, and everyone gets 20 free AI uses a day; past them, signed-in AI practice runs on your Bike4Mind credits.