Even if you consider the same question, I think we will get the answer to be infinity.
If we consider initial velocity of the bullet to be $v$, then its velocity after passing through first, second, ....$N$ plank will be
$$v(1-1/n) , v(1-1/n)^2, v(1-1/n)^3.....$$ respectively.
You must notice that velocity of the bullet ceases iff $(1-1/n)^N=0$, then for any value of $N$, the value won't be equal to $0$. Thus, $n$ must be equal to $1$ in order to satisfy the above condition.
If $n=1$, then the bullet losses $(1/1)$th of its velocity when passed through one plank, meaning its velocity remains constant inspite of passing through those planks. Thus, infinity number of planks is required to stop it.
Read this extracted paragraph from Feynman's "Surely You're Joking, Mr. Feynman!":
....Then cdtnes the list of problems. It says, "John
and his father go out to look at the stars. John sees two blue stars and a red star. His father sees a green star, a violet star, and two yellow stars. What
is the total temperat ure of the stars seen by John and his father?"--and I would explode in horror.
My wife would talk about the volcano downstairs. That's only an example: it was perpetually like that. Perpetual absurdity! There's no purpose
whatsoever in adding the temperature of two stars. Nobody ever does that except, maybe, to then take the average temperature of the stars, but not to
find out the total temperature of all the stars! It was awful! All it was was a game to get you to add, and they didn't understand what they were talking
about. It was like reading sentences with a few typographical errors, and then suddenly a whole sentence is written backwards. The mathematics was
like that. Just hopeless!......
Now you would understand why question is ambiguous (if the above calculation is right), no bullet will remain unaffected even if it passes though plank.