The tracks that appear on the home screen are the current one (top), and those that will follow, in order below it. There is no limit (that I know of) for number of tracks in a playlist - but the "tracks coming up list" only shows a certain number. When the repeat toggle is on, the track list wraps around to the beginning of the playlist. When you toggle it off, the "wrapped around" tracks (past the end of the playlist) will disappear, since they are no longer coming up.
OK, so the tracks in the giant playlist are there just not showing up on home screen.
And on a smaller one, after going through a few tracks, with repeat off, only the remainder are shown, but with repeat on, the ones that have already been played show back up at the end.