Response rate
The share of invited people who completed the survey.
Divide completed responses by invitations delivered. A 20% response rate on 1,000 invitations gives 200 responses. It is the number you need in order to plan recruitment, and the number a reader needs in order to judge the result: at 20%, four in five of the people you chose to ask are not in your data.
A low rate is not automatically a broken survey, and a high rate is not automatically a good one. What matters is whether the people who answered differ from the people who did not in ways that relate to what you are measuring. That is a question about who your non-responders are, and it cannot be fixed by collecting more of the same responders.
Report the response rate with the results so readers know how much of the invited sample took part.
Where it goes wrong
Confusing response rate with completion rate, which counts finishers as a share of people who started.