10^6 digits = 1,000,000 possibilities

125,000 accounts x 4 attempts per account per day = 500,000 attempts per day

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1-(1-1/1,000,000)^500,000 ≈ 39%

So every day they have a roughly 39% chance of success at 125,000 accounts.

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At a million accounts:

1-(1-1/1,000,000)^(4×1,000,000) ≈ 98%

Pretty close to 1 account per day

Off by a factor of 4 but the concept stands.

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And 125k accounts will be close to guaranteed to getting you one each week:

1-(1-1/1,000,000)^(7×4×125,000) ≈ 97%

Imagine the extreme case, where they pinged one million accounts and then tried the same code (123456) for each one. Statistically, 1 of those 1,000,000 six-digit TOTP codes will probably be 123456