From reading the documentation, this call returns a lost of followers. The default number of the list of followers it return is 20 thus you're getting the 20 contacts only. This API have a parameter called count where you can specify the number of followers you want to retrieve, the maximum is 200.

However there is a call that can reurn all the IDs for the followers: https://dev.twitter.com/rest/reference/get/followers/ids
This will return all followers IDs up to 5000 ID, however it will send ID only and not all the information of the followers. here is the documentation for this API: https://dev.twitter.com/rest/reference/get/followers/ids
Hope this helps!