Please create a cluster, enable HA and add all the hosts to that cluster, once done, select the cluster, now click on the summary tab in the right hand side pane. This will appear in 4.x VC onwards
on summary pane, Under VMware HA heading you will see option "Run Time Info". Click on this link it will give you the slot calculation and configuration for each slot. in the popup you will see available slots, these are the no. of VM's that you will be able to poweron in the environment with default settings given on the same popup.
Incase if you want to give more memory or CPU to any of the VM's the slot calculation will change, any time you give any VM more Memory or CPU than default, check the "Run Time Info" mentioned above, as that will give you remaining slots in the environment.