The atomic number of Silicon is 14. So there are 14 protons in the nucleus of every Silicon atom. But that's not the mass. Besides the protons, there are also some neutrons, and the atomic mass of the atom is the total number of (protons + neutrons) in its nucleus.
The number of neutrons is not the same in every Silicon atom. There are 3 stable isotopes of Silicon. A Silicon nucleus can have 14, 15 or 16 neutrons.
So an atom of Silicon can have an atomic mass of 28, 29, or 30.