As JPM said. There's probably a dished plate in there to. It'll be 1.5mm thick, with a dot punch mark just below the notch on the periphery of the disc.
You can't turn this plate around. It should go in after the first or second friction plate with the dot facing you.
Wear limit on the friction plates is 2.7mm. With the pack installed, you should have about 4mm of drum showing, as JPM said
Any more then you can build the pack thickness out using thicker plain plates.
TBH if it's a lot more then the clutch is ready for a change.
Either a stock set or a performance set............... even a 48T type
