A bullet loses (1/n)th of its velocity passing through one plank. The number of such planks that are required to stop the bullet can be:

Let the bullet be traveling at a speed v before entering the first plank.
After passing through the first plank, the velocity becomes v - v/n = v(n-1)/n.
After passing through the second plank, the velocity becomes v(n-1)/n - v(n-1)/(n^2) = v(n-1)^2/n^2.
After passing through k planks, the velocity becomes v((n-1)/n)^k.
The bullet stops when the velocity becomes 0. However, this will never happen since ((n-1)/n)^k will never equal zero for any finite k. Thus an infinite number of planks would be required to stop the bullet.
Therefore, the answer is Infinite.