rendering and saving onto desktop

My son is just beginning to learn/use Fusion 8 Beta. He's
hit a snag, however, and I'm wondering if you can help. When he rendered
the project he was working on, it saved but then his desktop suddenly had
hundreds of files on it. He says that he selected to save to his desktop
but as "single file." Is there a way to render & save but not have such a
large file load/dump?
Thanks,
Sarah Rabel

Sarah I just started learning Fusion myself but yes, there is a way to render to a single file if you choose a movie format as the output.

You can set both file type and a location to save the renders to from a saver node (right click / Add Tool / I/O / Saver).

In the Output format for the saver node you can select many different file formats - choose a video format (I'm on a Mac so the movie option for me is QuickTime; you may have different movie formats available if using Windows).

Do note, however, that a standard convention for most rendering and compositing apps is to output to an image format (Fusion seems to default to Targa files). Among other benefits, rendering to an image sequence
ensures one can continue a render from where one left off in the event of a system or program crash. Or in the event the movie file you might be outputting to gets corrupted. In such cases you do not loose an entire render job, just a portion of it.

Whatever file you output to, make sure it is saved with the correct file extension. Fusion doesn't seem to add these automatically when picking an output format. In addition to choosing a file type from the saver node's Output Format option, you need to select a file type / extension after clicking the browse button for the file name and path.