I recommend and read this article:

http://www.revespcardiol.org/es/revisiones-sistematicas-metaanalisis-bases-conceptuales/articulo/90024424/

Hello Fekri,

1. Follow the PRISMA statement and flow chart: http://prisma-statement.org

2. Be specific in your research question and objective (that way you'll know later on what to exclude and what to include).

3. The number of databases searched depends on your topic of interest, but not less than 2 databases.

I recommend the below article.

Arksey H, O’Malley L. 2005. Scoping studies: towards a methodological framework. International Journal of Social Research Methodology: Theory and Practice 8(1): 19–32. DOI: 10.1080/1364557032000119616.