The Library subscribes to many subject databases that will allow you to identify journal articles and other sources that will be useful for your review. You will need to search more than one to ensure a comprehensive and thorough search, as the goal is to consider all possible information of relevance. There is no magic number of databases which you should search - this may be guided by the amount of time you have to complete the review, as well as what databases are available to you, and which are relevant to your topic.
Consider your topic, and what databases would be good options to search. Looking at similar published reviews will be helpful, as part of the methodology of systematic reviews requires explicit statements around which databases have been searched. Also consider if there are other subject areas which may be equally relevant to search. For example, if your topic relates to nutrition programs in schools, you may wish to search databases related to Education, as well as Nutrition.
Depending on which databases you are searching, you may have to vary your keywords to reflect the particular requirements of a specific database. Some databases may use a controlled vocabulary, which will require you to use different search terms, e.g. MEDLINE
You can learn more about selecting databases and searching, and a detailed guide on how to develop your search strategy, incorporate Boolean logic and translate this into a database environment is available here.