What is the minimum number of research articles that must be cited to write a review article? I have extensively researched about a new area as part of my Doctoral research and want to write and publish a review article in some Science Citation Indexed (SCI) journal. But the problem is that there are only about 20 articles published till date  on this new area. I have seen many review papers on other areas published in SCI journals and found that normally the number of cited and referenced articles are quite high (sometimes even above 300 papers). And then there are non-SCI  journals where review papers are having very less number of cited and referenced articles (20-50 or more).
Your article needs to be interesting, the number of articles is secondary. If a field is very new than a review of a few articles could be interesting. However, if there are only 20 artcles it could also mean it is just too early to take stock, and your sub-sub-field is not ready for a review article yet. The purpose of a review article is to find partial answers on which there is some form of consensus, and identify open questions and new directions of research. Is your field sufficiently ripe for that?

Also look outside your discipline. It may be that there is a longstanding research tradition dealing with that issue in another discipline.