Question

How can elements with low atomic numbers have so many spectral lines?

Answer 1

There are actually an infinite number of lines, even with one electron. Not all of them are in the visible spectrum. Most involve very distant orbits, where the electrons are nearly free. That's just the way the quantum mechanics works. Very large atoms do tend to have more strong lines in a given visible wavelength interval, because of the larger number of electrons that can undergo transitions.