You are using Simple Survey and are curious about the limits for sending and receiving surveys.
Simple Survey itself doesn't apply specific limits on the number of survey invitations that can sent out each day or the number of responses that can be received.
Simple Survey relies on Salesforce for the actual sending of survey emails; Salesforce sets the email limits.
Per Salesforce documentation, The daily allocation for emails sent through email alerts is 1,000 per standard Salesforce license per org—except for free Developer Edition and trial orgs, where the daily workflow email allocation is 15. The overall org allocation is 2,000,000. This allocation applies to emails sent through email alerts in workflow rules, approval processes, flows, processes, or the REST API.