If you want to determine the number of protons , electrons or neutrons for any given element , then the periodic table is your best friend . The periodic table tells a lot of what you need to know about each element, including the number of electrons, protons and neutrons .

How to read an element cell and determining the number of subatomic particles in an neutral atom

The one (or two) big letter symbol is the element's symbol . Usually , the symbol includes the first letters of an element name although there are som exceptions . For example , here "Cl" is the element symbol for chlorine .
Above the symbol (the integer number) is the atomic number Z of the element which represents the number of protons ( or electrons if the atom is neutral) . For chlorine , Z = 17 .
Below the element ( the decimal number) is the atomic weight . The atomic weight is a weighted average of the mass of the isotopes of that element .This number is the sum of the number of protons and neutrons. It also tells us how many grams there is of an atom in a mole. For example , one mole of chlorine atoms would have a mass of 35.45 grams .
Atomic weight is given in atomic mass units (amu) .
Now , we know that a neutral chlorine atom has 17 protons and 17 electrons , but what about the number of neutrons ?
We can use the mass number M to find this .
M = Z + N => N = M - Z
For chlorine this means that the number of neutrons is 18.

Finding the number of subatomic particles in Ions

Ions are formed when an atom tend to either gain or lose electrons . Say we have Cl-. How many electrons, protons and neutrons this ion has?

As the written above , from reading the element cell we can tell that chlorine has 17 protons and 18 neutrons .
And since the charge is negative this means there must be more electrons than protons.
While a neutral atom for chlorine would have the same number of electrons as protons, the ion is shown to have a -1 charge. This means it has 1 more electron than the neutral atom : 17 + 1  = 18 electrons.
So , chlorine ion has 18 electrons .