Dear @Aziz Kara, so far I know there is no hard and fast rules regarding the minimum number of studies to be included in meta-analysis. This RG link maybe useful with similar topic.
"Is there any minimum number for studies that should be included in meta-analysis?" https://www.researchgate.net/post/Is-there-any-minimum-number-for-studies-that-should-be-included-in-meta-analysis
Proloy Barua, thank you very much for your answer. But I didn't meant meta-analysis, I meant meta-synthesis. There are diffferences between them. Meta-analysis is an quantitative while meta-synthesis is qualitative type.
There is no particular number of research that must be included in a systematic review, however if meta-analysis is desired, at least two studies must be included to summarize the data and synthesize the evidences that will offer some new evidence and information to present knowledge.
I hope this might help you in your endeavor: Article Qualitative meta-synthesis: a guide for the novice
And even though Proloy Barua refers to meta-analysis, I have to agree that also for a qualitative meta-synthesis there is no fixed set of rules on how many studies have to be included. Depending on your research interest the numbers can vary substantially.The main idea is rather that it captures all available attributes identified by research so far.