firebase-security-rules.json

{
  "rules": {
    ".read": true,
    ".write": false,
    "questions": {
      "$question_id": {
        "options": {
          "$opt_id": {
            "responseCount": {
              ".write": "auth !== null && auth.provider === 'anonymous' &&
                         newData.isNumber() && // make sure the new value is a number
                         (
                           (!data.exists() && newData.val() === 1) || // if responseCount doesn't exist for this option, only allow it to be set to 1
                           (newData.val() === data.val() + 1) // ensure that if responseCount does exist, the new value should only increment by 1
                         )"
            }
          }
        }
      }
    }
  }
}