How many electrons will each element gain or lose in forming an ion?

It depends on the number of valence electrons required to make octet or duplet( in case of H). For example, Nitrogen(atomic number = 7) has electronic configuration(2,5) which means nitrogen has 5 valence electrons and requires 3 more electrons to complete its octet. After gaining 3 electrons from atoms of an element with less electronegativity than N, it forms nitride ion (). Hope this helps.