feat: created proposed decision of max supported devices per atSign

decisions/2024-02-Devices-per-atSign.md

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+# Devices per atSign
+
+* **Status:** Draft
+* **Last Updated:** 2024-02-23
+* **Objective:** A standard supported maximum of devices per atSign to
+engineer to.
+
+## Context & Problem Statement
+
+With each atSign it is possible to have N devices associated and connected at
+any one time. With personal atSigns this is limited by the number of devices
+a person may have. But in an Enterprise setting an atSign could have N devices.
+With applications like SSH No Ports a company could have 10k devices all on
+one atSign.
+
+## Goals
+
+Decide on a reasonable number of devices supported simulataneously on one
+atSign and it associated atServer.
+
+### Non-goals
+
+We do not want to change the existing architecture and introduce complexity
+to reduce the risk of an a single atSign's atServer being offline and
+effecting huge numbers of devices in turn.
+
+## Other considerations
+
+Prevent a person using single atSign and connecting thousands of devices at
+low cost and high risk and putting stress on the rest of the infrastructure.
+
+* ### Option 1
+
+  Keep existing atServer configuration see at what point things fail and
+  decide on a % of that number of devices.
+
+## Proposal Summary
+
+Soft/advertised limit of 25 concurrent active devices per atSign
+
+## Proposal in Detail
+
+The existing limit of 200 TCP sessions and the existing limits of the docker
+container holding the atServer provide good support for up to 50 devices, so
+the suggested maximum should be 50% of that at 25 devices per atSign.
+This provides ample devices for most if not all use cases and limits risk to
+a maximum of 25 devices if an atServer fails whilst also protecting from
+potential abuse of over subscribed atSigns.