How do I reduce the number of hits?

If you get too many hits, the search must be limited. You need to analyse your search string and try to see what you can adjust to make it more limited.

You can add more search terms with AND between them. If you were searching for school AND bullying you could search for school AND bullying AND prevent.
You could search with more specific words. If you were searching for media you could search for social media.
You could do a phrase search and search for "social media" with quotation marks around it to get fewer but better hits.
You could try field searching or subject headings searching.

How do I increase the number of hits?

If you include too many terms from start, it may generate only a few documents, so try to remove one of the search terms.
You can also try truncation as a search technique, to search using various suffixes of a word. If you searched for child, search for child* .
Also consider if there are more general overriding terms or synonyms you can use for your search.
You can search using several synonyms at the same time by using the operator OR.
You can also try chain searching and citation searching, if you have found an article that you think is relevant.

When it comes to previous research sections in your essay or thesis, remember that all articles cannot illustrate all aspects of your subject and that it is your job to compile them to a unit. It can, therefore, be worth testing with two concepts at a time in different combinations.