Choose a style of clip art that is compatible with other images used in the document and compatible with the overall tone you want the document to convey. One large image packs more punch than a bunch of scattered icons. Don't forget about bullets, end signs, decorative dividers, and frames used in the document -- these add to the clip art count and can clutter the page if used excessively or without real purpose.
Make sure each image used in a piece serves a necessary purpose and is appropriate to the tone and style of the piece. When the image count rises above three on a single page, consider other ways to achieve the same effect without adding more visual stimulation.