Nouns that are "collective" (automatically suggesting a group) take a plural verb when the group as a whole is meant; they take a singular verb when the group can be thought of as individual members. Commonly used collective nouns include "number," "majority," "series," and "variety." Note that when collective nouns refer to a singular group as a whole they are often preceded by the word "a"; when they suggest individual group members they are often preceded by the word "the."
“A number of people were affected by the tragedy.”
— Special Cases in Subject Verb Agreement
PennState College of Earth & Mineral Sciences: John A Dutton e-Education Institute
Is this information wrong?
I found this article online and isn’t it the other way around? Should you use collective nouns that refer to a whole group with singular verbs and the individuals with plural verbs? Should the example above be “a number of people was affected by the tragedy?”