In this reaction, we can see that two iron atoms went from 3+ to neutral. This means they each gained three elections, in order to reduce their charge by three, meaning in total they gained six electrons. Likewise, there are three copper atoms that each went from neutral to +2. Their charge went up by two, meaning they each lost two electrons, meaning they lost six electrons in total. This means that six electrons went from copper to iron, meaning six electrons were exchanged.