This usually depends on the work invested by the authors in the research work. Generally only those involved in the direct planning and execution of the study and preparation of the manuscript are given authorship. The remaining can be acknowledged in the relevant section of the manuscript.
Hello Syed! I'm developing my PhD thesis and I'm also part of a research project. I can share my experience: in our project, the team members are distributed into different tasks. That means that our publications are not a product of every team members, so the number of authors is smaller than 8. That problem has only arisen regarding the project products such as poster presentations of the project itself - and the whole team members are authors (12 names).
This usually depends on the work invested by the authors in the research work. Generally only those involved in the direct planning and execution of the study and preparation of the manuscript are given authorship. The remaining can be acknowledged in the relevant section of the manuscript.
As far as I know the PubMed gives scope for listing as many as 35 authors. So upto that many can get authorship in a paper provided they have given intellectual contribution to the Research work. It is always good to acknowledge the intellectual contribution by giving authorship credit to them. Please do not forget that Research can not be done by one individual. It is always a team effort. So there is no harm in giving authorship credit to all those who have contributed by intellectual contribution. It is left to the main author to interpret this