1) go to a layout based on Delegates. Perform a find, specifying the Event and the Organization. The number of found records will be the number of Delegates.
You can also set up a summary report for a given event on the Delegates table sorted to group records by orgainization. Such a summary report can list the individual delegates under a sub heading with data from Organizations or, if you remove the body layout part and just use sub summary layout parts with summary fields, you can list each Organization as a single line in the report with a count of the number of Delegates.
Here's a link on summary reports you can take look at if interested: Creating Filemaker Pro summary reports--Tutorial