24m is the total sales of FFXIV. 15m for WoW was the last published concurrent subscribers in ~2014. These are not comparable numbers.
The current estimates are about 1.7m for FFXIV [1] and 1.2m for WoW [2] (+another 500k[3] if you include WoW classic, but there's some overlap between retail and classic). Since neither company releases subscriber counts that's our best guess.
It's clear FFXIV was ahead of WoW while FFXIV was undergoing it's endwalker pre-launch hype and did well to retain those players post-launch compared to prior expansions, while at the same time WoW was undergoing it's peak period of shadowlands discontent. But as WoW gets into dragonflight hype mode and FFXIV is between patches, it looks like WoW has drawn back even for a while.