I have almost 600 kits, going back to 2008...
Say whaaaaaa?
Even though most of their stores have closed and designers have long retired, I have a hard time letting go (though I have done a few mass purges through the years). And, with how quickly product moves and how often designers come and go, keeping a proper organization system is a moving target. I have a hard time keeping up!!
Right now I used a combo folder system and Lightroom to organize.
Folders are organized by shop -> designer -> kit
I keep one folder for previews and another for alpha previews.
I only keep the TIFF files of templates, they are organized by designer.
Then... I import all of my previews into Lightroom and organize them into smart collections. I tag each preview with relevant keywords like theme, color, major elements, etc. Templates are tagged with number of photos and single or double pages.
So, if I'm looking for a travel kit, or a spatula, or a purple flower, or a single template with 6 photos, it's a breeze to find just what I need!