Once the research question has been defined, the next step is the complete and unbiased identification of relevant studies, which will require conducting "a thorough, objective and reproducible search of a range of sources to identify as many relevant studies as possible (within resource limits)" (Cochrane handbook, Ch. 4.2.2).
"Searches for systematic reviews aim to be as extensive as possible ... It is ... necessary to strike a balance between striving for comprehensiveness and maintaining relevance when developing a search strategy". (Cochrane handbook, Ch. 4.4.3)
It is usual to have a lot of irrelevant articles in your search results.
Here is an example of a PRISMA flow chart.
There is no definitive answer to how sensitive your search should be, or how many results you screen.
You will have to make your own decision on how sensitive your search is.
See this guide from the Univerity of Toronto on how to develop a precise or sensitive search.