wich would be stronger an electromagnet wrapped by ten loops of wire around a nail or twenty?

Answer 1

Twenty.

Given the same amount of current, the "strength" of an electromagnet is proportional to the number of loops in it.

Another way to increase the magnetic field is to make the loops smaller.

Have fun, don't burn yourself, don't use wall current....
~Donkey Hotei

Answer 2

Twenty loops. More loops, more Magentic Flux. But wire size may need to be increased to keep Resistance same, so that current doesn't reduce.

Answer 3

Assuming same voltage, wire size, etc. 20.
More turns, more inductance.