In addition to quotas for questions, you can also set a maximum number of respondents for the entire survey. From the menu, go to 'Research' and 'Survey' . Click on the 'Extra' tab. Under 'Set maximum number of completes (quota)' enter the desired number.
Respondents who have started the survey can complete the survey. The maximum quota is exceeded; respondents will not be removed from the survey once they have started it.