During the writing of a review paper it is very difficult task to add number of authors. However it varies person to person and their contribution in paper. but, there shld be any ideal number for adding the authors?
It depends on the reviewing article ares, in my opinion. If authorial team is making review on some topic that is ''full'' of information, than it is logical that there should be more authors.
it depends on how interdisciplinary the area is and how deep the review intends to delve in the various areas. for instance a review on pretreatment in bioethanol production and other bioconversion research, may include a physicist to review NMR, X-ray crystallography and other data available on the effects of various pretreatments on biomass pretreatment; a microbiologist to review the yeasts used etc; a biochemist/microbial physiologist to accurately review the metabolic processes; a chemical engineer to talk fermentation and process design and control; a chemist for separation and purification processes and someone to discuss lifecycle analysis. this is all hypothetical of course.
on the other hand, it can just be limited to one or two, if they are subject matter expoerts or if it is a well-reviewed area etc
It all depends on how detailed the (lead) author(s) want(s) the review out to be.
Mostly Journal allows 5-7, but it can fluctuate till 8 to 9. it also depends upon the author's contribution statement, which justified the typical role of every co-author.