The absolute maximum# is 99 songs.
Most CDR's are 74min and most songs are about 3min.
So with some clear thinking, this results in 74min/3min = 24 songs.
When you want to put 99songs on a CD, this would result in 74min/99songs = 0.7min for the average song-length (not really realistic).
You can use programs like Easy CD pro (or Creator) to make your own compilations.
These programs start at 74min and when you add a song to the list, they count backwards.
eg: song1=3,5min => 74min-3,5min = 71,5min left.
It will give an error message if you put more songs in the list than the CDR can hold.