The numbers were by the Eiyuden developers during their kickstarter when they were estimating the budget for their game.
You can try to downplay the amount of assets in OT, but the sprite sheets don't lie (other than ambiguous standards on when to duplicate the left-right sprites on the sheet).
Here's one character/class combination for OT:
Nintendo Switch - Octopath Traveler - Olberic (Thief) - The #1 source for video game sprites on the internet!
www.spriters-resource.com
Here's a colelction for FF5.
SNES - Final Fantasy 5 (JPN) - Bartz Klauser - The #1 source for video game sprites on the internet!
www.spriters-resource.com
It is about 60 sprites per character/job in OT, vs. 10 sprites in FF5
Here's a comparison of the world map exploration / cutscene sprites:
Nintendo Switch - Octopath Traveler - Ophilia (Cleric / Base) - The #1 source for video game sprites on the internet!
www.spriters-resource.com
SNES - Final Fantasy 5 (JPN) - Bartz Klauser - The #1 source for video game sprites on the internet!
www.spriters-resource.com
Around 180 per each of the 8 characters in OT, vs. 27 for each of the 5 characters in FF5.
This is almost every single non-main character NPC in the entirety of FF5:
SNES - Final Fantasy 5 (JPN) - NPCs (Overworld) - The #1 source for video game sprites on the internet!
www.spriters-resource.com
Meanwhile, one NPC in OT:
Nintendo Switch - Octopath Traveler - Z'aanta - The #1 source for video game sprites on the internet!
www.spriters-resource.com
The spriters resource doesn't even have the full sets for OT, with there being a few hundred village NPCs that have full sprite sheets, because everybody can be fought or recruited.
www.reddit.com
Each of the 8 regions has a totally different set of NPCs and village terrains, filling 24 towns, which is far more than nearly any of S/E games of that era. It all adds up quickly multiplicatively as there's more regions, more unique characters per region, and more frames of animation per character in those regions. There's an overall diversity in NPCs is that fundamentally unrivaled by any game from the 16-bit era that it looks like at first glance. For battle sprites, having more distinct animations, like swinging different weapon types, or idle-idle, or waiting-idle, and giving each of those animations where a previous sprite sufficed in the past.
That people don't even recognize how much more elaborate the sprite-work is in Octopath Traveler than actual 16-bit games were speaks to how much people remember those older games through rose-tinted glasses.