When you are adding interests using your Facebook Ads Manager, Facebook will give you an indication / estimate of the total reach your ad will have.

Unfortunately, Facebook does not provide that data in their API, which simply means we don't have access to that information.

In other words, you cannot see the total reach / audience of the ad sets you are adding inside of ConnectExplore. You will have to add the interests to your Facebook Ads account first. Then, log into the Facebook Ads Manager to check the estimated reach there.