Factors determining number of turns in field winding of Sychronous generator
basically the limiting factors are the physical properties of copper, 
it's bulk resistivity, thermal conductivity, and tensile streength.

make the wire too thin and it'll snap, or be so long that you'll need an impractically high voltage to achieve the desired field,

make he wire too fat and you need too much current..

more turns of thinner wire don't otherwise get you a stronger magnetic field for less input because the thinner wire has a higher resistance so you end up burning the same amount of energy in the field winding either way.