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.

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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, 8 master balls in one game (Assuming you have one of each game). I'm not sure if the guy in Castelia City is in Black 2 and White 2 though. (If that's the case, maybe 6)

But in just white, only two.