Well, if the atom is neutral (and I must assume it is!) there are 20 electrons.
How do I know? I know that for the element calcium, #Z=20#, where #Z# is the #"atomic number"#, the number of positively charged, massive particles, that live in the calcium nucleus. If there are 20 positive charges, the #"neutral atom"# must contain 20 negative charges, 20 electrons, conceived to whizz about the calcium nucleus.
But why do I write #""^40Ca#? Well, this isotope has a mass of #"40 atomic mass units"# (note that the number of protons, i.e. the #"atomic number"#, has alreafy been specified by the #Ca# symbol, how?) We have have accounted for 20 of these massive particles in the protons. An additional 20 neutrons live in the nucleus and are responsible for the isotopic mass; neutrons are massive particles of neutral charge. If you have further questions, or want clarification, ax.