Like all Mainstream Pokemon Games, you won't be able to find every last Pokemon in existence in this game either. Actually, you will only be able to find the original 151 Kanto Pokemon, plus the two Brand New Mythical Pokemon. This means that all Johto (2nd Generation), Hoenn (3rd Generation), Sinnoh (4th Generation), Unova (5th Generation), Kalos (6th Generation), and Alola (7th Generation) Pokemon cannot be found within this game by no means whatsoever.
Furthermore, a player will not be able to find all 153 Available Pokemon in the Wild. This is due to Version Exclusives that sees certain Pokemon not being in one version to the next apart of the same set of Mainstream Pokemon Games, as well as because the three Mythical Pokemon (Mew, Meltan and Melmetal) must be obtained using alternative methods. Because not all 153 Kanto Pokedex Pokemon can be found in the Wild, players will need to either trade for the Pokemon they can't find within their version or transfer them from Pokemon GO.