A number of should be used with a plural verb. Think of it this way: A number of equates to the word they.

• A number of fans buy tickets at the game. (They buy tickets.) 
• A number of dogs in our neighborhood actually like the mailman. (They like the mailmam.)

On the other hand, when using the number of, use a singular verb. The number of equates to the word it.

• The number of snowstorms this year was more than expected. (It was more than expected.)
• The number of dogs in our neighborhood has grown over the past several years. (It has grown.)

Sherry