For a neutral atom, the number of electrons equals the atomic number
Find the atomic number of the element on the periodic table
Use that atomic number as the electron count
For a positive ion, subtract the charge from the atomic number
For a negative ion, add the charge to the atomic number
Example: sodium has atomic number 11, so a neutral sodium atom has 11 electrons
Example: chloride ion has atomic number 17 and a 1- charge, so it has 18 electrons