The formula (there is one) that determines the strength of an electromagnet is
H = I*N/L
H is the strength
I is the current
N is the number of turns
L is the length.
Question One
As N goes up, so does the strength of the electromagnet. The formula tells you this. If he wants a stronger magnet, he must Increase the number of turns.
The resistance of the wire will likely stay the same when you don't add any more turns. So if you increase the voltage, you will increase the current. That means you get a stronger magnet.