Wood planks appear to be able to stack up to 20 pieces, but it is only possible to stack to 20 planks when stacked inside of a fence's inventory.
Any number of planks >10 inside your inventory can accept another 10, making it possible to stack up to 19 planks inside a player's inventory.
Up to 10 planks can be stacked inside any other container.

This is very confusing and it is tedious to split the planks apart to maximize efficiency, the limit needs to actually be 20 everywhere, or just reduce it to 10 everywhere.