"My Number" is a 12-digit number assigned to individuals living in Japan under the social security, tax and measures against disasters.
If you are registered as a resident in Japan, your My Number (hereinafter called "Individual Number") is printed on the notification card that you receive from the ward office of your registered address. You must keep it securely.
Furthermore, since Individual Number has personal information data, you should not provide your Individual Number (Card) to others indiscriminately. Please take care concerning suspicious telephone calls, emails, letters and visits, etc. concerning the Individual Number.
You can make an application for your "My Number Card" (hereinafter called "Individual Number Card") via the website or by mail. It takes about one month from the application for Individual Number Card until the shipment of Notice of Issuance by municipalities.
About Individual Number
https://www.kojinbango-card.go.jp/en-mynumber/
If you have extended your period of stay, change your status of residence, or change your address, you need to update the information printed on your My Number Card at a ward office.
▶ Create a My Number Card and live a convenient life! (Immigration Services Agency of Japan website)