In addition to the quota on questions, you can also set a maximum number of respondents for the set up entire investigation. In the menu go to "Survey" and "Survey". Fill in the Extra options under "Research quotas (max. completes)" the desired number in.
Note: have respondents opened the survey and are still working on the questionnaire? Then they can complete the investigation. The maximum quota is then exceeded; respondents are not removed from the survey if they are started it.