https://wow.zamimg.com/uploads/blog/images/8774.png
The 543 at the top is just for that section, it's saying there are 543 helmet models available to collect and they have 4 currently. So if you click chest piece it'll have a different number. In total this makes literally thousands of slots saved from holding them about, and also lets you know what you haven't collected yet. For spoiler sake they could hide the visual of some items until you collect them if SE so chooses.
vid example:
https://youtu.be/J4Kwt40xEz4?t=13