You can see the amount of people you have reached and how many have opened your message and replied.
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You can find these performance metrics of your Engage flow on the top right part of the screen when you are in an Engage flow.

Here's what the numbers mean:

Reached: the amount of potential candidates who have received your email(s)

Opened: the amount of potential candidates who have opened one of your emails

Replied: the amount of potential candidates who have replied to one of your emails