If you are going shopping you should use the plural question.
Although when asking in plural form, we often add the word "any" in the question.
For example, "Do you have any bags?" or "do you have any dresses?" if you aren't sure if they have something.
If you are asking one person the question when not shopping, such as your friend if they have a bag so you can put something in it, you then should ask "Do you have a bag?".