Subscriptions listen for all events published under a specified prefix by devices on your Particle account and are not tied to individual devices. You do not need 5 subscriptions for 5 devices if they are all publishing with the same event prefix. When devices are publishing similar data it is best practice to use a single subscription handler instead of using several.
In your situation you could have the xenons publish events with identical prefixes but unique suffixes containing uniformly formatted payloads holding the sensor data. The argon would parse the suffixes in the subscription handler, allowing the argon to know which xenon is reporting the incoming sensor data, and then do whatever you wish with the sensor data.
As for why the number of Mesh subscriptions is capped at 5 is unknown to me. I’d imagine that 5 is the most stable limit the Particle Firmware Engineers found, and frankly if you need more than 5 subscriptions you are likely doing something wrong.
Also, you mention 3G potential but you are asking about the argon. Are you aware that the argon is the Wi-Fi + Mesh board?