Cash is recommended. Credit Cards have become way more popular in the lead up to (what was supposed to be the 2020) Olympics, but you will still see a lot of places that only accept cash.
Also make sure you have a coin purse as they use coins up to 500 yen, and you will have a LOT of coins. As far as how much yen, difficult to say, maybe 30,000 to 40,000 yen, and 7-Elevens all have ATM's you can use with western credit cards....I'm guessing other convenience stores also take them by now as well.