If I'm not mistaken, the author said at the time that the manga would be over 30 volumes, and the manga currently has 25, but he also said that with the 23 the final arc started ...
However, I agree with you
I made some estimations and the volumes of Fire Force usually contain nine chapters. Volume 23 started the "Final arc", so, checking what each volume contains from that volume onwards:
Volume 23: End of Obi's Rescue arc where "Moonlight Mask" saves Company 8 and Joker, the whole mini arc of Hibana, Karim and Foien checking the convent where Hibana and Iris were raised (+ Sumire reveal) and the beginning of Arthur's quest to forge a new Excalibur. (
Chapters 196-204).
Volume 24: End of Arthur's mini arc, beginning of the Stone Pillars arc. (
Chapters 205-213).
Volume 25: Whole trip of Shinra to the past of the world and the small 3 month time skip, ends with the appearence of the Doppelganger of Benimaru's master. (
Chapters 214-222).
Volume 26: Contains the fight of Beni and the Doppelganger of his master while Sho searches for the whereabouts of his father. (
Chapters 223-231).
We are currently in the middle of volume 27 as of chapter 236 (I imagine the volume will end in chapter 240). Volume 27 will probs cover Sho discovering the truth about his family and the battle against Titan Raffles, thus ending the "prologue" of the final arc. Volume 28 onwards will presumably be the start of the final battle as a whole. If Okubo sticks to his "over 30 volumes" estimation, I can see the series ending with 35 volumes more or less.