Wow merges every server in shards. FF only does in instances with servers from the same data center (think battlegroup). It is possible to visit other servers in the same data center.
Maybe i should note that when i say 50 it's just the ones i see in one given point. Of course it depends on where i am, but FF cities are very large. So, yes, there are way more than 100 people in the big cities in each given server. Probably more. Also that many on the gold saucer and Bozja/zednos. Heck even dozens of people just hanging out in the Gangos hub. These are basically the end game zone.
When you offer numbers, you have to offer the source. Guess i will go look.
So, mmo population tells a different story.
https://mmo-population.com/activity
FFXIV 2.8m
Wow retail 1.1m
WoW classic 730k
This for active players. Maybe you have a different source for your numbers? That is what i'm seeing.
Honestly, that makes sense with what i see in the games when playing them. But, there are no official numbers. Probably never will.