That doesn’t sound like a lot, but there were 644,000 more deaths than births (assuming an immigration rate of zero).
Every prefecture on mainland Japan saw a decline, so even internal migration was not a help anywhere.
Elon Musk saw this news and his saying that Japan will disappear was widely spread. He seems to be unaware of how this will soon be a global issue.
45 of Japan’s 47 prefectures have more residents aged 75 or older than those under 15
Source: https://www.nippon.com/en/japan-data/h01317/japan%E2%80%99s-population-drops-in-every-prefecture-except-okinawa.html