There is no set number of the databases to search for systematic reviews as each search is different and database selection should be guided by the review topic. However, searching between three to five databases is a good recommendation.
The fourth chapter of the Cochrane handbook focuses on searching for and selecting studies.
Google Scholar is not recommended as a replacement for searching across multiple databases because:
Separate database searching versus using the 'Find books and articles' search on the Library website:
Although the Library's Find books and articles search includes the content of most of the databases provided by the University of Lincoln library, the databases do need to be searched separately rather than using this search. This is because the 'Find books and articles' search doesn't facilitate controlled vocabulary term searching which needs to be done by using the subject headings/thesauri within each database. Searching each database separately is also key to the transparency and replicability of the systematic review process as you (and others) can clearly see which databases have been searched and the numbers of results retrieved from each one.
In systematic reviews, it is expected that you search each database separately using both controlled vocabulary and keyword searching and then replicate the search across each database searched. Controlled vocabulary searching involves using the thesaurus within the database to find the pre-set subject term for each keyword. Not all databases will have a thesaurus - Scopus for example does not.
If a database does not have a thesaurus, then only keyword searching can be undertaken.
Searching each database separately allows you to make the most of their advanced features and evaluate the results from each database. The University of Lincoln uses the EBSCO Host platform to host some of our academic databases so the interface will be the same (e.g. Medline, Cinahl and PsycINFO are all EBSCO Host databases).
Some key databases which could be used are listed below: