How many Pokémon could you catch in Diamond and Pearl?

Diamond and Pearl introduced 107 new Pokémon, bringing the world's total to 493, but the Sinnoh Pokédex you carry through the story holds only 151 of them, and the game asks just that you see 150, with Manaphy excused, before Professor Oak upgrades you to the National Pokédex. Seeing is easy. Owning is the real game, and it was designed to spill far beyond one cartridge.

Start with what Sinnoh itself provides beyond the story: daily swarms that rotate species onto random routes, a Great Marsh whose bins change every day, a Trophy Garden whose owner boasts a new rare guest each morning, and Poké Radar grass that shakes out species found nowhere else. Then the hardware joins in. A Game Boy Advance Pokémon game in the DS's second slot changes the wild population outright, and Pal Park imports six at a time from those cartridges permanently.

Even that leaves a residue only other products could fill. Manaphy could only arrive as an egg from Pokémon Ranger. The opposite version held its own exclusives and the other mascot. Darkrai, Shaymin and Arceus were not honestly catchable in Diamond and Pearl at all; their event items were only ever distributed for Platinum.

So the honest count is layered: 151 in the regional dex, a few hundred obtainable inside one well-equipped Sinnoh game, and a full 493 that functioned as a subscription to the whole DS-era ecosystem. Completion was the point, and completion was communal.