If an atom is in its natural state the number of protons will equal the number of electrons. This is because an atom naturally wants to be "even" and since protons have a positive charge, the atom will have the same number of electrons (negative) so that they cancel each other out.
The quick answer is 42 electrons would be required however, the question has a kicker in it, there should be between 42 and 50 neutrons there also if the atom is to be stable. with a marginal number of neutrons like 36 atom will be highly unstable and most likely self distruct .
