serial vs parallel for electromagnets: power consumption
ORIGINAL ANSWER:

If you connect 24 twelve volt electromagnets in series you'll need 288 volts to drive them properly, while if you connect them in parallel you'll need only 12 volts, which you already have, so it seems like parallel is the way to go.

EDITED ANSWER:

Since you've changed streams in mid-horse, it seems a little clarification is in order.

For identical cores, the strength of the electromagnet will  almost totally depend on the product (ampere turns) of the current through the coil times the number of turns wound about the core.

What that means is that if you have a core with a single turn wrapped about it and one ampere through the winding, (one ampere-turn) the strength of the magnetic field generated using that configuration will be identical to one generated by two turns wound about the core with half an ampere through the winding, which is also one ampere-turn.

Next, responding to your response to @Brian Drummond's comment, the steady-state math is no different for an inductive load than it is for a resistor since it all boils down to the resistance of the coil,

However, where the rubber meets the road, the impedance of the coil has to be accounted for when power is first connected to the coil, as does its inductive kickback when 
power is disconnected.

Then there's the resistance of the winding to consider, and all of a sudden the power required to generate the required magnetic field rears its ugly head.

Do you want to pursue this further?