Yes, unfortunately, there's not many reliable sources. From multiple sources, it seems that GW2 is roughly 1/3rd or 1/4th the size of FFXIV. That 100K active players is reliable number, so this should put FFXIV at around 200K-500K active players at any give time.
Which would get us at around 4% - 10% of players using mare. In case that mare statistic meant daily active players, and not active players at any given time, that would make it 0,8% to 2%. But just as others mentioned, exact numbers are not relevant, you just need to look in and outside the game to see what's happening.