A frame stack multiplies the per-frame pixel count by the number of frames, so more frames add more measured slots to the total. With 20 pixel slots per frame and 3 frames the stack holds 20 times 3 = 60 measured slots exactly for the stated toy detector ledger; real stacks have dropped frames, exposure variation, read noise, registration shifts, and measurement uncertainty.
Repeated detector frames multiply measured slots. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
One frame has checked slots
One detector frame has 20 pixel slots.
Nframe=20
Frames multiply measured slots
With 3 frames, the measured slot count is 60. Detector exposure, noise, and saturation are outside this count.