Each occurrence of a volunteer opportunity has a maximum number of sign-ups that are allowed, as defined in the occurrence record.

When an occurrence reaches capacity, volunteers are given the option of adding themselves to a wait list for that opportunity.

While you could create a volunteer opportunity for an event from its related list, this is the hard way. Instead use the volunteer opportunity wizard to create a volunteer opportunity that will take place during your event, and THEN associate it with the event.

The Volunteer Opportunity Overview gives you a 'dashboard' view of the status of the Opportunity and whether or not its been published.