20man - 20,000 yen

It’s not saved to the cloud because that’s not how the system works, it predates the cloud, and smart phones. It a very literal digital wallet, money is encoded into the card, rather than credited or debited from an account like a credit card. The system was designed for speed, no communications backhaul is required to verify a transaction when you tap in the gate, it’s just the card and the gate talking - maybe less of an issue today but this system was launched in 2001.

As for zeroing it out, you can charge the card to any value. For example Daimon to Narita costs 1344yen so if you want to leave with a zero balance - charge to exactly the last fare or purchase you’ll pay and you’ll end up with zero balance.