Under the form's Settings, set when it opens and closes, and how many responses it takes. Outside those times, or once it is full, posts get 403 with "name": "formClosedError" and a message you can show as it is; the hosted page shows it instead of the form. Spam does not take a place.
The Results tab counts the answers to choice fields (select and checkbox under Expected fields), with percentages. For encrypted forms the counting happens in your browser after you unlock the vault.