By definition all atoms have a neutral charge. This means that they have the same number of protons and electrons. Protons are positively charged and electrons are negatively charged. An equal number of positives and negatives will have a net charge of zero.
Each element in the periodic table has its own unique atomic number. That number specifies the number of protons in an atom. For example, Helium's atomic number is 2. That means it has 2 protons. Because the atom is neutral, it also has 2 electrons. Silicon has an atomic number of 14. That means it has 14 protons and 14 electrons.
Your question might also be asking about how to use the periodic table to determine the number of valence electrons in an atom. To do that, simply use the main group numbers. Main group numbers indicate an entire column of atoms. Hydrogen's column is group 1. Boron's is group 3. Oxygen's is group 6. Whatever that group number is, that is the number of valence electrons. For example: Bromine is element number 35. It has 35 electrons. It is found in main group 7, so it has 7 valence electrons.