hi,
The (Vs/Vp= Ns/Np = Ip/Is) are ratio's.
If you have say a 240Vac step down to 12Vac transformer, you should get 12Vac on the output winding.
The important point is the VA , volt/ampere rating of the transformer.
For an ideal say, 12VA transformer you could draw 1amp from the 12V secondary, the primary current would be 1*[ 12/240] = 0.05A
For a 24VA transformer you could draw 2amps from the 12V secondary, the primary current would be 2 * [12/240] = 0.1A
OK.?