Hmm. I suspect I'm really inefficient with multiBLD because of how I do it. For various reasons, I decided to do all corners of a cube at one locus, then all edges of a cube at the next locus. So I store an entire 3x3x3 in 2 loci. I have 9 loci in a room, but don't use the last one for multi, so I can store 4 cubes in a room.
I guess that's a little unfair because I usually put flipped edges or twisted corners on the ceiling of the locus, hanging over the main set of images. So maybe you could consider those extra loci, depending on your point of view.
But anyway that can actually lead to 8 images at a single locus for edges, in the worst possible case, although I doubt I've ever had worse than 7.
It certainly doesn't feel stressed to me for multi at 4 images per locus. 5 maybe starts to feel strained. But it is nice that 3 images can approximate person-action-object (even though I don't really do PAO, I am sometimes kind of close with it), and so those tend to be easier to remember.
So I decided on 3 for the poll, figuring that's probably more like what most people would consider not yet stressed.