8.
It was a beast and I went through blades lickity split. I was careful to keep the shape cut from sliding, made several passes until I couldn't hear the fabric cut anymore. Still had to snip some threads with the scizzor tip.
4 wasn't real bad and I prefer 2.
I have to depend on my shape cut because I tend to let a long edge ruler slide when I am making the pass and I tend to rub the ruler with the rotary blade, ruining the blade and ruler.

4 with the 45 and 8 with the 60. But always with a new blade!

Usually 2, sometimes 4, but not more. It kind of depends on the fabric and what size/type piece or strip I'm cutting. Even though all my fabric is 100% cotton, certain ones lend themselves well to cutting 4 layers at once, others not so much. I also attempt to get more use out of a cutting blade than I should; having a new blade in the cutter makes a difference.

I have cut 8 with a dull blade and it was a mess. I can do 8 easy with a new/ sharp blade. I try to stick with 4-5 depending on what shape i am doing. if you can sharpen or have a new blade i would recommend it for more than 2 layers.

I've done six but I prefer four.

interesting thread, thanks!