A Plus subscription allows you to view up to 5,000 responses per form or survey at any point in time. To receive more than 5,000 responses, we recommend exporting the data in your response table (File->Export Responses), saving it locally in one of the supported formats (.pdf, .csv, or .xls) and then deleting it from the response table in FormsCentral. Once you free up space in your response table you will be able to collect additional responses. Repeating these steps will allow you to collect unlimited responses with a Plus subscription.
Note: FormsCentral actually stores up to 7,500 responses internally - we provide a buffer of 2,500 responses to make sure you do not lose data if you unknowingly went over 5,000. But if you exceed 7,500 submissions then respondents will not be able to submit their forms (they will get an error) until you export and delete your responses to make more room for responses.