The number of errors on yahoo.com reaches a level that hasn’t been seen since yesterday at noon. Here’s one more:
A number of these errors are avoidable by simply employing an editor with a thorough understanding of grammar.
The American Heritage Dictionary describes how to choose the correct verb to use with the noun number:
As a collective noun number may take either a singular or a plural verb. It takes a singular verb when it is preceded by the definite article the: The number of skilled workers is increasing. It takes a plural verb when preceded by the indefinite article a: A number of the workers have learned new skills.