Good question @chelseys! The Ring Alarm System does require a Base Station in order for the other Ring Alarm devices to operate and communicate with your Ring app. It sounds like you will only be needing one of our Ring Alarm 5 piece start kits. This kit will include a Base Station, 1 Contact Sensor, 1 Motion Detector, 1 Keypad, and 1 Range Extender.
If you are only looking for simple activity alerts, and do not want emergency dispatches or professional monitoring, you are able to register your Ring Alarm for self-monitoring. While self-monitoring, you can receive open and closed sensor alerts to your mobile device and Ring app, as well as use armed modes which will still trigger the siren if a sensor is tripped while armed. Just keep in mind, if your Alarm siren is triggered while self-monitoring, there is no dispatch or monitoring station calls, like there would be if professionally monitored. I hope this helps!