I don’t understand the 8-event limit related to Aggregated Event Measurement. Is that per pixel? Per ad account? What does it include?

The 8-event limit will apply to each domain and includes standard pixel events and custom conversions. You can have multiple pixels and ad accounts connected to a single domain, but you can’t optimize for more than eight events. Read More: 8-Event Limit and Domain Verification Scenarios: iOS 14 Challenges

How will targeting be impacted by privacy changes related to iOS 14? Will it impact custom audiences? What about interest targeting?

All indications are that this shouldn’t impact interest targeting. Attribution, which is only related to ad reporting, also shouldn’t apply to website custom audiences. If a user opts not to be tracked, however, they should be expected to be removed from audiences. Read More: Facebook Advertising Audiences and iOS 14