It really depends on the format of the book. Based on a maybe not-so-random selection of the books I’ve read (my bookshelf on bookmeter), I used jpdb to find the total amount of characters in the whole book, then I divided it by the amount of pages the book has (from bookmeter) to find an estimate.
| Book | Characters | Pages | Char/Page |
|---|---|---|---|
| Honzuki 1 | 158 457 | 351 | 451 |
| Sword Art Online 1 | 141 359 | 352 | 402 |
| Konbini Ningen | 60 188 | 168 | 358 |
| Kiki’s Delivery Service | 84 180 | 256 | 329 |
| Watashi no Shiawase na Kekkon 1 | 94 572 | 288 | 328 |
| Kusuriya no Hitorigoto 1 | 106 624 | 328 | 325 |
So the average based on this tiny sample would be 366 characters per page :3