https://twitter.com/Kaphotics/status/1193896437978808320Beyond the known count in the Galar Dex, there are 35 other species that have stats and models (which explains Melmetal sprite). Kanto & Alola starters, and some legendaries. 435 species available in total!
https://pastebin.com/3jNGBZqhGameFreak has scrubbed all other entries for species outside the 435, which includes any potential Mythicals (event). But they forgot to change the max species ID in the code, hinting that there are 2 new Mythicals in the future (DLC?)
