I just use one folder to store all of my attachments. This may seem disorganized, but with the power of internal linking in Obsidian I have no concern about them all being dumped there. I run with the assumption that I will be able to find every file I need in that folder based on links or the search features in Obsidian.
There are many other ideas of how you could use more folders for organization. However, I recommend aligning with @nickmilo approach that says to minimize folders and rely on links. That is the approach I use for my vault.