"A number of..."

     [This is a fast-access FAQ excerpt.]
"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.