It is said that atoms cannot be created. If so, then how did atoms get created after the Big Bang?
Atoms can be created and destroyed.  For example, in nuclear fusion two or more atoms make one atom.  In nuclear fission, one atom makes two or more atoms.


  how did the atoms form after the Big Bang? 


The theory of this process in known as Big Bang Nucleosynthesis



Starting from protons and neutrons, light elements formed.  Later, in stars, nuclei up to iron formed through nuclear fusion.  Nuclei heavier than iron formed from supernovae by the r-process and asymptotic branch giant stars by the s-process. 


  Also, does this mean that the number of atoms in our universe has remained the same since then? 


No.  The number constantly changes due to fusion, fission, supernovae, creation of neutron stars, black holes and possibily quark stars. 


  Also, what happens as the universe expands? Does the atoms just scatter or are new atoms being created?


Currently, this process does not create or destroy atoms.  However, according to the theory of accelerating expansion, there is a senario referred to as the "big rip" in which all atoms would be destroyed.