An electrically neutral atom must have:
Same number of protons and electrons
Protons are positively charged, electrons are negatively charged and neutrons don't have any charge.
Since the magnitude of charge on an electron and a proton are equal but opposite in sign i.e. positive and negative.
So an atom is said to be electrically neutral when it has the same number of protons and electrons and the number of neutrons doesn't affect the charge of an atom.