Look carefully and you'll see that only three Daleks are ever moving at any one time, this is because there were only three Dalek operators in residence, because normally there were only ever three functioning Daleks. A fourth was made available especially for this story, the Dalek Supreme, borrowed from Terry Nation's collection, but all the rest are 'goons', lifeless but posable background props that don't move.
Terry Nation was unsure how many Dalek casings would be available to the production team, so he wrote around them. He originally envisioned both Rebec and Codal masquerading as Daleks, for instance.
The Doctor reverses the polarity, of the recording device.
This episode was watched by 10.7 million viewers on its original transmission.