12 episodes

I don't know. if I had to guess, they will probably cover six LNs because each of the heroes gets their own LN dedicated to finding them (I believe), there were two that I didn't think had a ton of substance and then theres the phoenix Naofumi mentioned at the end of S2. as an assumption from some moments in S2 and promo materials for S3, naofumi will probably meet two important characters shown at the end of S2, two characters are going to die and all the cardinal heroes in Raphtalia's world will get some major weapon unlocks. 12 episodes seems a bit rushed if they intend to cover all of the material I think they will cover, but they also might condense or scrub some material to make it work

Can't they end S3 at volume 12 and continue to volume 15 in a simpler Season 4 ?

totally agreeing with you on how terrible of an idea it would be to put six novels in 12/13 episodes. there were two novels that I thought were fluff material that they could make into their own episodes and have the remaining 10/11 episodes for the other novels, but if they do go that way, the remaining four novels would still likely be rushed. my prediction is that we'll see the cardinal stooges, we'll see the slaves from the end of s2, we'll see the phoenix naofumi was talking about and what Ost meant with not needing the wrath shield, on top of seeing the character in the promo poster in action. also, i know the tortoise arc wasn't a favorite arc of many readers. if they made it 12 eps just to get through it and go back to 25 eps, I can understand that