This answer seems relevant for you, as it describes the Redis behavior.
But as you're mentioning AWS ElastiCache, max amount of connections seems to be up to 65 000 as referenced in this answer. Answer contains link to AWS documentation.
Amount of connections from your web application (php, python, ruby, ...) to Redis server is what you're looking for, the size of the node is more about "how much you can store", not "how many connections you can handle".
So this might be your "rule of thumb": https://medium.com/appaloosa-store-engineering/you-should-revise-your-redis-max-connections-setting-8136f063c916