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.
What happens if I hit the limit or go over the limit?
If the number of responses hit the limit (only applicable to monthly licenses) you'll billed at the rate of $0.10 / Response to the credit card on file. If you have a survey running, DATA COLLECTION WILL NOT STOP.