I understand that the repolarization is because some K+ chanels are opened with the action potential. But whats happens with the sodium chemical gradient that is needed for a new action potential? Are there further Na-K pumps in the axon membrane?
Yes, there are Na-K pumps all over.
However, it is important to recognize that ion concentrations change very little during action potentials. The change in membrane potential is due to changes in ion conductances, very few ions actually flow. It is still necessary to maintain concentration gradients over many many action potentials, but the change in concentrations during the course of a single action potential is negligible.