Say H atoms are excited to 4th level, n=4 that is, how many lines do we see? How to decide the number of the lines?
An excited state electron may transition to any lower level. From n=4, the electon could go to n=3, n=2 or n=1. Of these 3 transitions, only n=4 to n=2 (wavelength 486nm) is visible light. n=4 to n=1 is ultraviolet and n=4 to n=3 is infrared.
The wavelengths of the transitions are given by the Rydberg formula.