The heading for paragraph 7 of Harbrace College Handbook,
Chapter 6a states, When regarded as a unit, collective nouns, as
well as noun phrases denoting quantity, take singular verbs. . .
. The number is singular; a number is plural. "The number of
students was small." [The number is taken as a unit.] "A number
of students were taking tests." [A number refers to
individuals.] Their examples indicate that it's different
depending on if the number is taken as a unit or refers to
individuals. In your case I think it's individuals and should
use the plural.