It has become something of a trend over the past year or so to note the increasing amount of co-authorship on scientific papers (with perhaps a hint of concern this might not be a good thing). Having hundreds or even thousands of authors on a single paper does seem a bit perverse, and of course there is the question of where it all might end. But to be honest, it was not all that long ago that having just five authors on a paper would have seemed perverse, and so in considering limitations might I suggest there is only one obvious place to draw a line. For whether it is five authors or five thousand, the real concern is why no one goes it alone. Have scientists become so utterly unsure of themselves that they now feel obligated to always bring a gang.
Group think is group mediocrity, and I can think of no phrase that better describes the current realm of science than group mediocrity.