Reply rate is the share of messages you send that get a response. If you send 20 cold emails and 4 people reply, your reply rate is 20%. In job-search outreach, reply rate is the metric that matters most, because a reply — even a "not right now" — is the start of a relationship, while silence is nothing.
Reply rate is driven far more by personalization, relevance, and reaching a verified, correct recipient than by volume. A small batch of sharply targeted emails to real decision-makers routinely beats a large blast of generic ones. A strong follow-up sequence also lifts reply rate substantially, since many responses come from the second or third touch rather than the first.
Don't confuse reply rate with open rate. An opened email that gets no response told you nothing useful; a reply is the only outcome that moves you toward a conversation.