How many starter Pokémon does a player choose from at the beginning of each core Pokémon game?

How to get close to this number

The type triangle is the answer. Grass beats Water, Water beats Fire, Fire beats Grass — a rock-paper-scissors cycle needs exactly three options, so the starter count is fixed by the balance mechanic rather than chosen for variety.

That is why the number has never changed across nine generations while almost everything else in the series has. A mechanic built on a three-way cycle cannot accommodate a fourth starter without breaking the symmetry, so the constraint is mathematical and not traditional.

It is also a genuinely good piece of design for a first decision. Three options is enough to feel like a meaningful choice and few enough that a child can evaluate them all, and the cycle guarantees no option is strictly best. A first choice that is consequential, comprehensible and balanced is harder to design than it looks.