This is the TOTAL number of responses across all surveys that you may have for this upgrade level.
Note: One completed survey = one response. If your respondents take the survey more than once, then each of those will be counted as a separate response.
Also, a response is a counted only when a respondent answers all questions and then submits the survey. In a branched survey (involving multiple submits) one completed survey is considered a response.
For monthly billing, this is the maximum number of responses that you can collect across all surveys.
What happens if I hit the limit or go over the limit?
If the number of responses hit the limit or go over the limit, you'll have to upgrade to the next subscription level. If you have a survey running DATA COLLECTION WILL NOT STOP. However you will not have access to your data until you upgrade your license.