Can you catch all 151 on one cartridge?

No — and here is exactly how far one copy of Red, Blue or Yellow gets on its own, what each version can never offer, and the smallest setup that finishes the dex.

Not on Red, not on Blue, not on Yellow, and not with any amount of patience. A single cartridge played by one person tops out at 125 of the 151 on Red or Blue, and 130 on Yellow.

Those numbers already assume you never make a mistake. Here is where the rest of them go.

Two footnotes on that table, both of which make the real figure slightly worse. Obtainable counts Mew, which exists only through a glitch — play the game as written and every total drops by one, to 124, 124 and 129. And spent on choices assumes you make each of those choices as well as it can be made; pick the same fossil on two cartridges and you do worse.

What each version simply does not have

Twenty-eight of the 151 are missing from at least one version. Everything not listed here is available in all three.

Six of them exist in exactly one version. Ekans, Arbok and Electabuzz are Red’s alone; Meowth, Persian and Magmar are Blue’s. Nothing you can do on Yellow produces any of the six.

Yellow is the odd one out in both directions. It is missing the most — twelve, including the entire Weedle and Koffing lines and all three of Jynx, Electabuzz and Magmar — and yet it finishes ahead of the other two, because it hands you all three Kanto starters as gifts instead of making you pick one.

The four that need a second Game Boy

These four exist in every version and cannot be obtained by anyone playing alone. Each evolves from its previous form the moment it is traded, and there is no level, stone or item that will do it instead.

The trade does not have to be permanent. Send Kadabra across, let it evolve into Alakazam on the other cartridge, and trade it straight back — both cartridges register Alakazam, and you keep it.

The choices you only get to make once

Each of these is a one-off. Take one and the others are gone for that cartridge, no matter how long you play it.

This is where Yellow’s advantage comes from, and it is worth being precise about what it costs elsewhere: on Red or Blue, choosing a starter forfeits six species, because you lose two full three-stage lines rather than two Pokémon.

So why is it possible at all?

Because the Pokédex remembers what has passed through, not what you are holding.

The owned flag is set the moment a species is registered, and it stays set after you trade that Pokémon away. Nothing checks up on you later — the flag is one bit in a nineteen-byte array, and nothing in the game ever goes back to clear it. Finishing the dex on one cartridge therefore does not mean owning all 151 at once — it means each of the 151 being registered there once, however briefly.

That is what turns an impossible goal into a logistics problem. Every gap above is closed by borrowing from another cartridge, registering the species, and sending it home.

The smallest setup that finishes it

Two cartridges, two Game Boys and a link cable, and the two cartridges have to be the right pair:

Red and Blue is the only pair that covers all 151. Add Yellow to either one and three species are still unreachable, because Yellow shares its gaps with whichever of the two you picked.

Two cartridges are enough for availability, but not quite for the choices. Both the starter and the Eevee groups have three options, and each playthrough can take only one — so a second cartridge leaves you one starter line and one Eevee evolution short. Closing that needs a third playthrough: a third cartridge, or restarting one of the two after its Pokémon are safely elsewhere.

Three playthroughs, two of them abbreviated, is the practical answer. That is the shape of the problem, and planning the trades around it — who catches what, which cartridge holds the dex, what still has to come home — is most of the work in finishing a Gen 1 dex.

The battling half of those playthroughs answers to a different table entirely — the Gen 1 type chart, which is not the one modern guides print.

And Mew

Mew was never distributed in a way most players could reach, and it is not in any version’s encounter tables. It is, however, reliably obtainable on an unmodified cartridge through a glitch in how the game handles a trainer’s line of sight — no cheat device, no second copy, and no risk to the save.

It has one requirement that has to be met early: certain trainers must be left unfought, hours before the glitch itself becomes relevant. Start a Gen 1 dex without knowing that, and Mew is the one gap a third cartridge cannot close.