How many slits for a discrete spectrum?

[Drizzy]

Homework Statement

I watched two videos on KhanAcademy, one was about light interference with 2 slits and the other was with 4 slits. The video with 2 slits got a continuous spectrum whereas the one with 4 slits got a discrete spectrum. So my question is: how many slits does it take to get a discrete spectrum and why that number specifically?

Homework Equations

The Attempt at a Solution

 

[Simon Bridge]

Citation please.

The spectra should be as discrete or continuous as the source.
ie. if a white light source, the diffraction grating should result in a series of rainbow patterns.