Transformer Designing
For any given core area and frequency, there is a maximum 'volts per turn'. 

For a typical small (50VA, ish?) toroidal core I have to hand, the core is 25mm by 13mm. Iron saturates somewhere between 1.5T and 2T depending on the grade. If I run the core with the flux peaking at +/- 1.8T at 50Hz, I will generate about 170mV peak per turn. So a 12Vrms winding would need 100 turns, the 240v mains winding would need 2000.

It is possible to use a core large enough to get the voltage in a single turn. For instance, an iron core with an area of 0.032m, that is 0.13m by 0.25m, the size of a railway sleeper, would give you 12Vrms per turn at 50Hz, and a very heavy transformer.

As Sam points out, increasing the frequency increases the volts per turn proportionally. This is why aircraft use 400Hz AC, it reduces the weight of iron and copper in a transformer by an order of magnitude, and is about the highest frequency practical with iron laminations. Ferrite transformers used in SMPS tend to use frequencies in the 10s to 100s of kHz, and these can get away with one or a few turns.