How do I know how many planks I have?

In order to get the Worse than Ghosts and Goblins achievements, I need to collect exactly 9 of the strange planks. My plan is to skip all of the ones from one of the acts while collecting all of them from the others. I usually don't play the entire single-player game in one sitting and there's a possibility that I may have chosen to skip the planks from a previous act or maybe I intended to do this on the last act, but I just can't remember and I may have accidentally missed a plank somewhere anyways. I don't want to have to start over because I screwed this up again so it would be a great benefit to know how many strange planks I have collected before I do the last act. Is there a way to check how many planks I have? Maybe by inspecting the save file or through something in the interface that I missed?

No. There is no way to check how many planks you have, or the work to check would spend more time than playing through the game. I might get some downvotes but this is the answer. Because they are considered secret so you cannot check that in-game.

The save is basically unreadable without certain knowledges of computers, otherwise you will edit it instead of playing through this exciting games.

I would suggest you just take a note whenever you get a plank, in this way you don't have to purposely skip all planks in an act, and you can choose the planks that is easier to obtain.

Good luck.