What is the minimum number of research articles that must be cited to write a review article?
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.