Why do elements produce different numbers of spectral lines

Elements can be differentiated on the basis of their spectral lines. As the spectra of any element is obtained when the electrons in the orbits of an atom change their states. When they move from lower to higher orbits they absorbs the radiation or energy. When the fall back from higher energy state to lower state they emit the radiations. Thats how the spectra of radiation is obtained. Now coming towards your question, as the different elements have different atomic numbers, they have different number of electrons in their shells. That's why the presence of the number of electrons in the shells of different elements vary. So the different number of electrons in different elements lead to the production of different numbers of spectral lines.