Mass conversion of .numbers files to .csv
Automator is the tool for the job.

You can do a "watch me do" and have it export an already open file to a new folder as csv. Then stop recording the script.

At that point, you can add an acton to process files and folders so the script will one by one open each file, save it to the new location. As long as the names aren't conflicting, you would have an easy tool.

Better would be to script the actual export options, but start simple and then add the batch processing.