Define "group authorship". Then, check

https://www.councilscienceeditors.org/resource-library/editorial-policies/cse-policies/approved-by-the-cse-board-of-directors/cse-recommendations-for-group-author-articles-in-scientific-journals-and-bibliometric-databases/

Mohamed - it depends on context. The minimum is obviously two - to move from individual to 'group'. This is a minimum in terms of usually low-complexity studies whereby two authors are preferable to one for cross-checking of data etc. For more complex studies i.e. longitudinal, international, competetive grant-funded etc - then a broader team would be expected. That said, and has often been discussed on RG, one starts questioning the roles and contributions of research teams when there are more than 10 researchers.

In high energy physics, there seems to be no limit. See, e.g., this paper with more than 2300 authors: Article Search for an excited lepton that decays via a contact inter...

Here is a bigger one (5154 authors):

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjwrNmijY7qAhXLo4sKHX5xDygQFjABegQIChAE&url=https%3A%2F%2Fwww.nature.com%2Fnews%2Fphysics-paper-sets-record-with-more-than-5-000-authors-1.17567&usg=AOvVaw1pevkI7Su9RkQjTkIqXJqV

In some cases the publisher might impose a ceiling on the maximum number of authors that can be included in a single article.

Clinical trials usually have a significant number of authors, and I do not think they have a maximum. Many times they end by adding the name of a research group collectively to complete the list. Details are given at the end of the paper.

It depends on the nature of the project conducted. Though we have just one author for some researches, still we can find many cases where more than one author collaborated in conducting a project or a study. Though academic collaboration is praised in the field of research, yet I myself have a reservation regarding a big number of authors for humble study.