I am working on a project to install 100+ nodes of temperature sensors in an area, all of which perform the same function. The data they publish is the sensor id and the reading
Faced with a similar dilemma and the impracticality of creating too many 'things' on the AWS IoT administration console; I've done some research and found that connecting multiple devices to the same 'thing' is strongly discouraged by AWS.
Anyway its not possible to keep two different nodes using the same MQTT id connected to the same thing (the last connected node with the same id kicks out the previously connected node), although you can use your client ID with the help of some code.
I learned that actually you don't need a 'thing' to connect to AWS IoT, just a certificate will do; and that you can create elements on AWS IoT service by code.
So, in summary; facing a similar question myself, I ran across this information below, found it useful in my case and sharing it here. https://forums.aws.amazon.com/thread.jspa?threadID=234102