Question

The number of _______ in an atom of an element is the element's atomic number

Answer 1

The number of protons in an atom is the atom's atomic number.
In a neutral atom the number of protons is equal to the number of electrons.
There is possibility of losing the electrons, but the number of protons are not loosed, except in nuclear chemistry.
So the number of protons is equal to the atom's atomic number.