The subpages linked here detail some numbers and requirements linked to completing the Pokédex in Legends: Arceus.
Unlike other Pokémon games, Legends: Arceus actually makes it possible to get all Pokémon without trading. This is also the ultimate goal of the Professor Oak Challenge.
In Mystery Dungeon, I often strive to recruit as many Pokémon as possible at the evolutionary stage I want them at, no matter their spawn rate, and evolve only the ones I can't get otherwise. This page outlines what Pokémon can be obtained with a similar approach to Legends: Arceus.
Although all Pokémon can be obtained, not all forms can be obtained at one time. This page discusses the highest possible number of Pokémon that can be obtained in a living dex, and what would be required to finish the living dex.