This article is from the alt.usage.english FAQ, by Mark Israel misrael@scripps.edu with numerous contributions by others.
"A number of ..." usually requires a plural verb. In "A number
of employees were present", it's the employees who were present, not
the number. "A number of" is just a fuzzy quantifier. ("A number
of..." may need a singular in the much rarer contexts where it does
not function as a quantifier: "A number of this magnitude requires
5 bytes to store.")
On the other hand, "the number of..." always takes the singular:
"The number of employees who were present was small." Here, it's
the number that was small, not the employees.
 
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