This is false.
Ukraine had 48 million at the time of its last census in 2001
It has already had the lowest birth-rate in Europe, so population was already declining.
EU sources indicate it has had outward migration of 300-500K per year over that period
It lost 8+ million when Russia took Crimea
in 2022, 8 million fled West while about 2 million fled East
It lost another 6-7 million when Russia took Donbas/Luhansk.
Rump Ukraine is lucky to have 20 million remaining
Stop making up numbers. Crimea TOTAL population in 2021 was 2.4 million.