The atomic number for lithium is 3. An atom must possess 3 protons to be called a lithium atom. If an atom has four protons, it will not be considered lithium but instead beryllium. Any atom without 3 protons is not considered lithium.
To be neutrally charged, the amount of electrons must equal the number of protons in an atom. The number of electrons must also be 3.
Depending on the isotope, however, the number and type of neutrons may vary. An isotope is a specific kind of lithium. You can have lithium-6 which has 3 neutrons. Lithium-7 would contain 4 neutrons, while lithium-8 would contain 5 neutrons.
It is easy to determine the number neutrons present in an atom by looking at its atomic mass, and subtracting the number protons. If your atom has anatomic mass of 5 and you know there are 3 protons, you can subtract 3 form 5. This will give you 2. This will tell you that there are 2 neutrons. This is because protons as well as neutrons have approximately 1 amu each. However, electrons are very small and don’t often affect many calculations.