How to perform FFT for I(real) & Q(imaginary) arrays?
At each loop of the Cooley-Tukey algorithm, the butterfly diagram of the Cooley-Tukey algorithm always has the same number of inputs to the butterfly diagram as the number of inputs in that loop.

Looking at the graphic in your question, we see there are 8 inputs.  The butterfly diagram is the right half of your picture which has 8 inputs, 4 labeled E and 4 labeled O.  Now to find E and O we have to do two individual instances of the Cooley-Tukey algorithm loop and these will have N/2=4 inputs each, so the butterfly diagrams in these instances will be have 4 inputs.