Let's see here.. Assuming it's like the average dex books, there will be about... 60-100 pages for index, battle info, etc, and the rest for pokemon. Lately, they've split it for 2 pokemon a page, with extra forms getting their spot on a page too, so... with what we currently know is in the dex... including the credible leaks... that's roughly 255 pokemon alone (including forms), but with four pokemon per 2 pages (but putting Gourgeist and Pumpkaboo forms together and assuming Frillish, Jellicent, Shellos, and Gastrodon forms aren't separated), that makes alone about 64 pages. If there's 448 pages and they do 4 pokemon per page, that would give us potentially around 128 pages for pokedex. Let's double that for a sec and get 255 pages under the assumption of us only knowing exactly half the dex. This gives us around 510 pokemon total. But, still, that's only 193 spare pages. If my guess of roughly 100 extra pages for non pokedex is correct, that would mean roughly 180 pokemon left unaccounted for.
So, in the end I'd say the pokemon count will be somewhere between 510 and 680 if you include forms.
