In singles, anywhere from 40,000 (in a quiet week) to 100,000+. A strong number one might sell 200,000 in a week, but it doesn't necessarily have to. It all depends on how many the record at number two sells, really...
It also depends on if it is the singles chart or album chart. As the singles chart is country wide sales but with albums some companies such as HMV go by their company sales for the weekly number one album rather than the overall country sales.
I think the record for the lowest number of sales to get to number 1 was Iron Maiden - Bring Your Daughter To The Slaughter with around 27,000, the highest not to go to number one was Wham, with Last Christmas, which had well over 100,000 sales.