Question

How many master balls can you get in white?

Comments

if you can  get more than one (i know where to get the first one at)

Answer 1

2.
>V Black and White Opelucid City Meet Professor Juniper in town after earning all eight Unova Badges.
V Black and White Castelia City Pokémon Center Talk to a man after the player has outsider Pokémon with at least 50 different original Trainers. The man will no longer give out gifts after the Master Ball.

Bulbapedia

Comments

You can actually get infinite, can't you? From Join Avenue?

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This is in the original White, where Join Avenue didn't exist. It was just a regular gate.

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Do they have to be different trainers that you trade with?

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with at least 50 different original trainers.

read. the. answer.

Answer 2

I know you probably want to have master balls in other gen. 5 games, but if you trade them when attached to a Pokemon, get one after collecting 8 gym badges, and trading with 50 people then talking to the guy in Castelia City, you could get at most, 6 master balls in one game (Assuming you have both White and Black). (With Black 2 and White 2, Join Avenue's Raffle Shop may let you get multiple Master Balls.)

But in just white, only two.