A mysterious confluence of combinatorics and bad DIY instructions. It says and shows 12 of part D but I only have 6. Why? Solution to follow
In reply to @christianp
Solution: holes a, b, c and d are mutually exclusive options for where the middle shelf goes. So you only need 6 of part D
In reply to @christianp
If there had been fewer options, there wouldn't coincidentally been as many potential holes as screws in the pack (there are two sides)