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 can go to the "Manage" page of a particular survey and click on the survey you'd like to increase the sample for. On the right hand side of the page, it will show you the targeting you've selected, click the three dots on the top of the page (next to the status) and this will give you the option to Increase sample & Duplicate.

If you choose to increase your sample, you 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.

Using this feature keeps your results in one single dashboard and one export. If you need the additional responses to be presented separately, duplicate and re-run the survey instead. Find out how to duplicate surveys here.