Just in case you do not remember what My Number is, I explain it here. Most of you should have received My Number from a government office when you registered as a resident in Japan. My Number is a 12-digit number used for tax payment and other processes. A My Number card is an ID card that shows My Number.
You do not get a My Number card unless you apply for it, so more than 80% of Japanese people do not have it in 2020.
I assume most of foreign nationals do not have the card either, so you will probably apply for the money by postal mail.