Would you find the number of electrons when given atomic number, mass number, and number of protons and neutrons?
Here are the rules that you MUST remember.
(1) The atomic number is always equivalent to the number of protons.
(2) The atomic mass is the sum of number of protons and number of neutrons.
(3) For atoms that are neutral (no charge), the number of protons is equal to number of electrons. Otherwise, the number of electrons decreases if the atom has a positive charge (+) and increases if it has a negative charge (-).
So, even if you are only given the chemical symbol, charge (if there is any) and the atomic number, it is still possible to know the number of electrons.