A body of work is comprised of multiple pieces that are cohesive in nature. It usually doesn’t represent an artist’s oeuvre, a word that most often refers to an artist’s lifetime of work.
The definition of a body of work varies from artist to artist. For one artist, a body of work might be defined by size. For another artist, it might be color, media, or subject matter.
I can't give you a precise number of pieces that are within a body of work. I can't even give you a number on average.
The emerging artist is most concerned with producing a body of work defined by a recognizable style. This means that the emerging artist is looking for quantity as well as quality. A few pieces by a new artist won’t convince the art establishment of your capabilities. You need enough for a solid exhibition.
A more established artist might define a body of work by subject matter. Think of Claude Monet’s Waterlilies or photographer Larry Clark’s Tulsa series. (Note: To confuse things further, a series can be a body of work, but a body of work can also be made up of multiple series.)
Bodies of work are often clearly defined on artists' websites.
Plein-air painter Joellyn Duesberry defines her bodies of work by location.
Mixed-media artist Kesha Bruce does a great job defining (through text) how each of her series are different.
How will you know when you have a body of work?
I think you will know when you’ve hit upon a defined body of work. You feel confident in the quality, proud of the results, and ready to share it with the world. Until you feel this way, keep making art.