In a consistent sense via constant releases as of now? Top 8 at least. The series only started to really take off after Nintendo for once properly marketed the series with Awakening so it's not that high. Looking at sales in general, it's not even listed in the top 10. It's basically a late bloomer series.
Certainly in top 10. I guess Pokémon, FF, DQ are the biggest. Would assume Kingdom Hearts Probably has more sales and maybe the Mario RPGs, but those are shifting away from RPG elements or have no studio to make them.
Well that's a hard metric to judge. Obviously it's behind Pokemon, FF, DQ, and probably Kingdom Hearts, so it's at least in 5th place I'd say. Maybe 6th going by singular game sales since I think Three Houses sold less than Persona 5 (maybe only when counting R though).
But it's definitely one of the most consistently well-selling JRPG series and the team is probably fully aware of that, which would line up with how the next entry is supposedly fairly ambitious.
Wikipedia has a list of the most profitable as a whole series, but to me it's probably more useful to look at how the most recent entry or two has done. Or at least that's a fun thing to look into IMO!
Off the top of my head (with some wiki diving to confirm numbers, which are rough numbers based on what I could find quickly) and going by most recent mainline entries (some of these like FF7R + Intergrade/Persona 5+Royal/DQ11 + DQ11S include multiple versions (and Pokemon obviously)):
#1 Pokemon: is in its own league with its 25+ million sellers (Sword and Shield most recently), even Arceus and BDSP are over 12 million
#2 Final Fantasy: 15 sold over 10 million, and FF7R is harder to pin down but is at like 7-8 million?
#3 Kingdom Hearts: KH3 sold over 6.7 million
#4 Dragon Quest: 11 sold over 6.5 million
#5 Persona: P5 sold over 5 million
#6
Fire Emblem: Three Houses sold over 3.84 million
#7 Paper Mario: Origami King is maybe iffy as a JRPG but sold 3.34 million
#8 Octopath Traveler: Sold over 2.5 million (I love that this is #8)
#9 Xenoblade: XC2 sold over 2.4 million, too early to judge XC3
#10 Tales: Tales of Arise sold over 2 million
Obviously this list is very rough, with minimal research, and only based on recent games and not whole series. It also doesn't include some things that would be way up in the list that are iffy as JRPGs in my mind but you can add them in there too: Elden Ring has sold a bajillion (over 16.6 million) copies, Nier Automata is at 6.5 million, Monster Hunter Rise has sold over 10.3 million.
So if you include those three, Fire Emblem is #9, but either way it's Top 10.