There are many reasons why you might want to increase the number of respondents who take your survey.
Whatever your reasons are, it's easy to increase the size of the sample for your survey. Whether your survey is live and actively collecting results, or has already closed, you click on "Manage" at the top right of your results dashboard and click "add respondents" .
This will open up a pop-up where you can select the wave and audience you want to add respondents for. Then simply input the number of additional responses required, select the credit pot you wish to use, and then click the Increase Sample button in the pop up box.
Keep in mind that only the survey owner is able to add respondents for a survey. You can transfer ownership of a survey if needed.