Planks and metal sheets show as 10/20. So I knew you could stack to 20. but putting 10 in your hands and trying to combine with another 10 stack doesn't work.
I found a work around by adding 20 of either to a structure in the process of being built, then removing from the structure. Though it doesn't do much good when you're already at the structure.
The same is true for logs. Can't manually combine more than 2. But using the workaround above can get you up to 4.