1 - Prepare a lot of planks
2 - Put up to 9 planks on your inventory (let´s call it stack A)
3 - Group up to 9 planks on your hands (let´s call it stack B)
4 - Drop the stack B
5 - Group up to 9 planks on your hands (let´s call it stack C)
6 - Combine stack B with stack C on your hands.
7 - Drop the 18 stack above stack A.
8 - Carry up to 100 planks on a field backpack (or 200 if you take another full backpack on your hands)

By experience, I think the game blocks piling up planks on the inventory when it reaches 10. The same happens in the hands slot. But combining less than that in hands with planks from the ground can produce a pile over 10 planks in your hands and that can be combined again with the smaller inventory stack to a limit of 20 per pile.