The max number of results you can get in a single request is 100, which could you get by adding ?limit=100 to the end of that url.
If you have more than 100 followers you have to make multiple requests and either increase the offset querystring parameter by the same amount as the limit, or by taking the _cursor value from one set of results, and using that as the cursor querystring parameter which will get the next page of results, and repeat the process until all followers are obtained.