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Amazon has the SWSH Pokedex guidebook listed, and it's 448 pages.
For comparison:
>SM Pokedex guidebook - 384 pages - 301 Pokemon
>USUM Pokedex guidebook - 720 pages - 809 Pokemon
These books tend to have two Pokemon per page including megas and forms. They also tend to have about 200 to 270 pages of moves, items, checklists, art, developer interviews, post game guide, etc.
Given this, let's go with a minimum of 200 non-dex pages. That leaves 248 pages for Pokemon. Multiply that by two and we get 496 dex entries. Given this is a rough estimate, we can assume the Galar dex will be somewhere between the mid 400s and low 500s including.
TL;DR: Expect SWSH to have about 450 to 500 Pokemon.
