Collective nouns related to quantity (percentage and fractions—thirty percent, one-fourth, half) take a singular verb when preceded by the. Otherwise, the verb agrees with the number of the noun in the prepositional phrase that follows it:
After receiving their pink slips this week, the third of employees with stock options has decided to cash in company stocks.
Fortunately, four fifths of the employees have stocks to cash in. (plural noun in prepositional phrase)
Unfortunately, four fifths of last year’s harvest was lost. (singular noun in prep phrase)
“Number of” as a collective noun
Whether it takes a singular or plural verb depends on which article precedes it: definite the or indefinite a.
The number of trees planted this year has doubled.
A number of experts have demonstrated that planting trees in the fall improves their viability.
“One of” takes a singular verb because it refers to "one."
One of those men fixes cars every day.
One of those who
“One of those who” takes a plural verb because the verb refers to “those”
One of those men who fix cars will work on your new project.