The best rule of thumb is “try it, and if you need more resources, get bigger instances, and if the instance is basically idle, get smaller instances”. Users is a terrible metric for sizing, BTW; page views is much better, particularly absolute peak page views per second. If you’re using auto-scaling you can cheat a bit and just use lots of little instances and have them appear and disappear, but that’s a lot more work (by like a couple of orders of magnitude) than just spinning up an instance per AZ and heading off to the pub.
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