Overclockers stack up to 4 each. You only have four slots. Assume that by 8 overclockers you need a capacitor and transformer upgrade. You've now used all four slots.
(Yes, you can avoid the transformer upgrade through the creative use of multiple LV-transformers to feed a single machine. I'm not going to explain how to do that.)
(Yes, you can avoid the transformer upgrade through the creative use of multiple LV-transformers to feed a single machine. I'm not going to explain how to do that.)