A single postcode is normally allocated to each street. (So there might be 50 or so houses, or just a single house, with the same postcode).
The principal exceptions are
(i) major businesses, which generally have their own postcodes, separate to other buildings in the same street ; and
(ii) very long roads, where different areas might be allocated to different postmen. (In such cases it's common for opposite sides of the road to have different postcodes but the postcodes can also change, on either or both sides of the road, as you progress along the street).
So an isolated cottage might be the only property with that postcode. But (from looking at the delivery lists for our local Yellow Pages, which are here beside my desk) I can find streets where around 100 properties have the same postcode.
Chris