Hi Mart
It depends what it is for, if mulching weeds then two layers would be good, but the more layers you apply the less moisture may penetrate.
If I were using it to cover couch grass, for example, I would apply two layers and keep enough in reserve for another layer or two in spring, placed on top of what youy lay now when it is starting to develop holes which weeds grow through. At that point, worms love the lower layer.
You can also put it in the compost heap! Good in summer especially, mixed with green matter, but good now too.
In case any one is wondering, I only remommend cardboarding where weeds need mulching. I am not applying it here on my garden which is weed free, although today (a different matter) we were flooded:
and this bed is planted with garlic and broad beans!