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 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.
You can collect unlimited responses in your Essentials account, however each survey is limited to a maximum of 300 responses.
What happens if I hit the limit or go over the limit?
If the number of responses hit the limit, the survey will continue to collect responses, however to access the results of the survey you will have to upgrade your account to a paid license. DATA COLLECTION WILL NOT STOP.