What would a 2012r2 vm really need for Exchange 2013 (standard, only machine, simple mbx/cas role) for about 25 users?
The docs I have read want 2-4CPU and 8-12GB, I think thats really a lot.
I'll often give vm's a lot of vCPU/ram upon console setup, after that, I strip them down once I've moved them into the auto startups.
As they get used, I keep my eye on them in vCenter and see if I need to readjust, but rarely.
I've got a ton of little single-role 2012r2 vm's that run really well with 1vCPU and 1GB, like file server, cal server, sql...
It depends... 😉
Why not start with the minimal spec and then do what you always do: keep an eye on the VM. Be sure to enable the hot-add and hot-plug options for cpu and ram.
Exchange Server is an application that consumes high CPU and Memory due to its structure. Because of this, 2-4 CPUs may even come at a minimum. If the exchange is used heavily, you may even have to increase it. Note, however, that if you give more CPU, CPU Ready Time occurs. In this case, the virtual machine starts to run less performance. My advice is to start with 4 vCPU, 16GB Memory and reserve these resources. Exchange Server services are affected by resource bottlenecks very quickly and can crash.