How do I reserve RAM on the Hyper-V 2012 host partition for VMs only?

Solution 1:

Source

300 MB for the Hypervisor plus 32 MB for the first GB of RAM allocated to each virtual machine plus another 8 MB for every additional GB of RAM allocated to each virtual machine plus 512 MB for the host operating system running on the root partition

As a general rule of thumb, plan to allocate at least 512 MB memory for the root partition to provide services such as I/O virtualization, snapshot files support, and child partition management.

300+32+(45*8)=692MB For the first VM

692+32+(45*8)=1084 for both VMs

1084+512=1596MB total accounting the rule of thumb

So this would be just the hypervisor consumption. Now considering that you have more than 5 GB used even when the first VM is provisioned. I would suggest to look at the output of tasklist in command prompt and look for server processes consuming RAM