I still think that's being generous. OG Hyrule Warriors and DQ Heroes had what, a dozen playable characters total?
If you count the free DLC characters (who were basically half implemented in the game at launch), then Hyrule Warriors had 16. However do keep in mind that Hyrule Warriors had 23 move sets at launch since Link and Lana had a bunch of extras. The translation of the most recent scans with info on the game indicated that FE Warriors will have the most playable characters for a first iteration of a crossover Warriors game. So depending on what they are counting when they say that, that could mean 14+, 17+, or 24+. I think low 20s is reasonable based on the info we have at the moment. Although I would not be shocked (or disappointed) if it ends up being 18.
Hell we have a dozen characters just based on the info in the current scans. We have the two OCs. We know the five wielders from the launch trailer will make it in. Then we know we have a bow user, axe user, spear user, and mage. There is at least one horse user, but Xander covers that. Probably have a flyer, but no guarantee that the flyer won't be the only axe user or only spear user, so that does not add any more necessarily. And there will almost certainly be an OC villain. So totaling that up gives us a floor of...12. Of which, we have 7 sword users. So glad they didn't just do all lords since there would be too many sword users then. >.>
Don't forget that a lot of Fate's Royals covers a lot of the series' weapon and class types. For Classes, you have a Manakete, a Dancer, a Swordmaster, a Paladin, an Archer, a Dark Knight, a Wyvern Rider, a Pegasus Knight, a Troubadour and a Cleric; and covers Swords 4 times, Lances three times, Magic potentially 4 times, Bows up to three times, Axes once, Staffs up to 4 times, and one potential Hidden Weapon user (or three if Corrin's servants get involved with +2 Staffs). And that's not even touching Awakening or Shadow Dragon/Mystery of the Emblem.
And also, while Hyrule Warriors only had 16 playable characters initially, that figure was coming from a series with essentially 1 playable character (although I suppose Phantom Zelda might also be considered playable, but she didn't really get in so YMMV).
Dragon Quest Heroes is a more fair look at what Koei Temco's gonna do here and even then, its still a series with 35 non-protagonist PCs to pull from across 9 games. IIRC, Sacred Stones has more characters than that alone (if you count the Creature Campaign units) and that has one of the smallest casts in the series.
Anyways, I fully expect an Extreme/4-2/Legends/"whatever fancy thing they're calling the expansion this time" to add more characters eventually, but I also expect palletswap shenanigans like TMS did with folks like Abel, Lon'qu, the two Aurelis Knights, and the Whitewing Sisters.
Snark Edit: Also, failed to mention this but Seigbert and Noire are totally confirmed
(sorta/kinda/in a palletswapped-ish way).
Actual Edit: Ignoring SSJ Seig and Vanilla flavored Noire for a moment; Lucina, Lissa and Male Robin were accidentally confirmed with the reveal of Special Edition box. Whoops. At least now we have our first non-Sword user (and also M!Robin who uses a Tome, in addition to a Sword; at least neither are Prfs, even if the series has been trying to play up Thoron as M!Robin's Prf), and if they go with War Cleric, then we now have our first Axe user.