In PS4 Original mostly you’ve got 2 sets of thermal pads 1 set on top side of the board on VRAM and another set on the bottom side of the board on VRAM.
I haven’t actually seen them stacked and I would rather say they either fell off as they usually do. Or maybe you have misplaced them?
Sometimes in other versions of the console they are placed on different components as well such as Renesas IC (that mostly can be seen in PS4 Slim edition).
About the thickness it is indeed 1mm but I wouldn’t worry as long as you’ve got them on all VRAMs plus it has to make contact between VRAM and shield.
I personally have used the thermal pads from one of my ps4 slim consoles in order to add to customers console.Later I have just simply replaced them with 2mm thermal pads as they were cheaper to get than 1mm and those ones I got are providing similar thermal conductivity performance.
To simply put, don’t worry too much just 1 of each on VRAM will do fine as long as they have contact with the shield.