The atomic number of an atom is the number of

The atomic number or (also called) the proton number, represents the number of protons, which also represents the number of electrons in that single atom itself (an atom which has not gone through reactions of any kind will have the same number of electrons as number of protons).

Hope this helps! :)

P.s. This question seems more of a Chemistry than Biology question in my opinion

Atomic number = Number of protons
Mass number = Number of protons and neutrons