According statistics published in PubMed, the average number of coauthors per paper was 1.56 in 1950, meaning at least half of papers were single authored at that time. This number now reaches to 4.77 in 2008. The age of publishing a paper alone is gone.
In extreme cases, the maximal number of coauthors each year is always bigger than 600 in the past 8 years (2001 to 2008). The all time winner is a paper with 743 coauthors published in 2001 (Aubert et al, 2001, Phys Rev Lett). This paper has 8 pages in total; author list and affiliation takes 3 pages.
Another trend is the increasing use of collective names. For example, Global Alliance for Women’s Health, or Mended Hearts, Inc appear in the author list of a 2007 paper (Mosca et al).
Not surprisingly, the number of authors also increases steadily. The number of indexed authors (including duplications) per year reaches to 3 million in 2007, compared to 1.3 million in 1990. Competition is serious!
Refer to: http://www.nlm.nih.gov/bsd/authors1.html
