VLM 5000 VM sizing



We have two VLM5000 (version 7.2.42). They load balance connections to 12 Exchange 2016 servers.

Currently our Exchange 2016 servers host +14000 mailboxes.
But in the upcoming weeks they will host +27000 mailboxes.

Each VLM5000 VM had the mimimal requirements (2 vCpu and 2 GB RAM).
Last week both VM crashed (Out of memory warning, then CPU stuck at 100%).

To solve this we had to stop VM then changed VM configuration to 4 vCpu and 4GB memory.

I wonder if there is some "sizing" guidance regarding VM.
I undersand/guess/see that a VLM5000 with "more" CPU and "more" memory will handle more connections.

But how much memory of vCPU is required to maintain 27000 maiboxes?

For 27000 mailboxes we expect +100'000 connections to the Kemp VLM5000.