The atomic number is the number of protons in the nucleus so is different for each element. E.g helium is written
The bottom number is the atomic number, also known as the proton number. The top number is the mass number and is equal to the sum of the protons + the neutrons in the nucleus.
These are the particles that make up the mass of the atom.
There is no need to learn atomic numbers. They are given on any periodic table.
In this table the atomic number is the number above the symbol. You'll notice it is also the order of the elements in the table. This was how atomic number was originally defined before we knew about protons, neutrons and electrons.