The answer is "What song?" Some songs have many, many chords, but there are hundreds of songs that have only 3 chords, and hundreds more that have only 4 chords.
So to be able to play a simple song like "Amazing Grace" (simple in terms of music, not theology) or "On Top of Old Smoky" or "Happy Birthday" all you really need to know are 3 chords -- the tonic chord, the subdominant chord, and the dominant chord.
Please see "Songs you can play with just 4 chords" for exciting information on this idea.
Also see "Songs With 3 or 4 Chords"
