The number of protons in an atom equals the number of what?
The number protons equals the number of electrons in a NEUTRAL atom.
Protons are conceived to be positively charged, massive nuclear particles. Electrons are conceived to be particles of negligible mass that orbit the nuclear core. Clearly, for a neutral atom, the number of electrons must equal the number of nuclear protons. The number of nuclear protons,
Now an atom can gain or lose electrons to form a negatively or positively charged ion; it cannot lose nuclear protons because it that case the identity of the nucleus changes.