The papers I've seen that have more than a dozen or so authors often work like this: The first and second authors were the project drivers, and did the writeup; the names in the middle were involved in data collection, number-cruching or sub-projects, and are often distributed across different organizations; and the last author (or two) will be a very well-known name, someone who has basically blessed the project by granting their name to it as an established researcher in the area, but who most likely only advised the main researchers.
In general it only counts on your CV if you're the first, second, or maybe third author. But a project with 20 people on it would probably collapse under the organizational weight before completion if it wasn't a pretty big deal. For example, the papers on the Human Genome Project and shotgun sequencing had a lot of authors, with the breakdown as described above.
In general it only counts on your CV if you're the first, second, or maybe third author. But a project with 20 people on it would probably collapse under the organizational weight before completion if it wasn't a pretty big deal. For example, the papers on the Human Genome Project and shotgun sequencing had a lot of authors, with the breakdown as described above.