If you apply an AC voltage to a coil a flux will be generated.
If a coil is "exposed" to flux lines there's going to be a voltage across it.
This is explained by the Faraday Equation:
$$\mathrm{V_t = N \ \frac{d\Phi}{dt}}$$
NOTE: I deliberately omitted the negative sign.
From the equation above:
- If you apply \$\mathrm{V_t}\$ across an N-turn winding it'll generate a changing flux, \$\mathrm{d\Phi/dt}\$.
- A changing magnetic flux of \$\mathrm{d\Phi/dt}\$ generates a voltage of \$\mathrm{V_t}\$ across an N-turn winding.
Consider a multiple-winding transformer with a magnetic core (e.g. ferrite). If you apply an AC voltage across one winding, it'll generate a changing flux. The flux lines will be "carried" or "transferred" (for the sake of simplicity) along the magnetic core. Any other windings that are "exposed" to these flux lines will generate voltage, regardless of the total number of windings.
Of course, the generated voltage will be proportional to the number of turns, as per the equation above (Saturation and other non-linear effects are ignored for the sake of simplicity).
For further details, consider studying "magnetic circuits".
