""The number of turns decides the voltage you get. The size of wire determines the resistance and is one factor in determining the current you get.

Normally you would choose the thickest wire that will go into the space and give the chosen number of turns. Sometimes the ideal wire size is too thick, either to handle physically or with these alternators you run the risk of eddy loss in very thick wires.

In these cases you can make up the same cross sectional area of copper by using more than one strand of smaller wire and it behaves as if you used the thicker wire but without the problems mentioned. There is no actual gain or advantage except where I have mentioned about the limitations of very thick wire.

Sometimes it is done for other convenience reasons. If you ideally needed #12 but you had a reel of #15 then using 2 in hand #15 would do exactly the same job.""

Here is the whole thread,