Circuits, Voltage, Resistance, Current - Physics 101 / AP Physics Review with Dianna Cowern

Here's what I don't understand.  When wires move thru a magnetic field, electric current is produced.  They say this is an interaction of the electrons of the copper atoms.  OK so the first batch of electrons are stripped from copper atoms and the 2nd and the 3rd, etc.  Why don't the copper atom's run out of electrons?  Where do the electrons after the 4th batch, etc come from?  This is the part I don't get?