In the Projects module, you have access to the Audience Overlap section. It is provided by an external provider and is available for Instagram, YouTube and TikTok.

It is the percentage of unique and overlapping followers for your selection of influencers. Unique followers are followers who are only part of one influencer's audience.

There's no recommended percentage for this metric, it completely depends on your campaign strategy (awareness, engagement etc...): Do you want to reach as many people as possible but display content fewer times? Or do you prefer to display content many times but reach a smaller amount of people? It's up to you!