Using you example of an 8 node cluster 1ith 132 GB total - for a 4 GB VM with a 2 GB reservation there will need to be 2 GB of free RAM on a single host for that VM to start in the cluster - so now let us assume that each host uses 4 GB of RAM that means there will be 100 GB of RAM available for VMs - so theoretically using the above VM configuration you will only be able to start 50 VMs in the cluster.
Now lets work through 2 resource pools - one RP (RP1) with 20 GB reservation and the other with 40 GB (RP2) reservation leaving 40 GB available to the cluster - the other key is that the resource pools are configured without expandable reservations - so this means that when a machine is powered in will reserve its 2 GB from the reservation in the resource pool - this means that in RP1 you will be able to start 10 VMs and in RP2 you will be able start 20 VMs - because the RPs are configured to not use expandable reservations 40GB that are available to the cluster will not be used. If expandable reservations are enabled the VM will be able to use the additional cluster memory.