If your aim is average then each layer needs to have a progressive value based on previous layers in the stack.
layer no alpha (%)
And so on. Proof:
If you want to have a stack of 4 layers and each have same contribution then: At layer 2 each layer has a 50 50 because layer 2 lets half trough. At layer 3 layer one is 33 and layers 1 and 2 contribute 67% of 50 Which is 33% within rounding error. At layer 4 layer layer 4 has 25 % and layers 1-3 have 75% or 33 which again is 25% withing rounding error.
Now the problem with this is that for many layers its tedious to set up. But also there is a significant source of rounding errors happening at each layer computation.
Now since you have tagged this Adobe Photoshop, theres a easier and less rounding prone way. What you do is:
- You select the layers you want to average in the layers panel.
- Right click on the layers palette and choose make Convert to smart object
- Now there is a special function in Layer → Smart Objects → Stack Mode → Mean
It also has other nifty statistical tools. Benefit of this is that if you want a eliminate you can just hide it and it does not partake in the stack. Also it does less rounding errors. But more importantly you canb use the super useful median and min/max. (although the others have their uses for scientific imaging)
How many can you stack? Well it depends on the images and how they overlap it is possible that a average of a couple of hunred images can tell you something, i have used such images. But more likely blending thisway more than 12 images wont give you much.