Start with the number of esx/esxi hosts. In this example, there are 3 esxi hosts.
You can see that in the "Total hosts in cluster" line. The next line shows you all three hosts are currently running.
Now look at the number of virtual machines running. In this case the number is 6. This is easy to see and notice that you see that value twice (Used slots and Total powered on vms in cluster).
Look at the size of the memory and cpu reservations of all of the machines and document the highest one. If no reservations are present, the defaults are 256mhzs and 0mbs of ram. This has been changed in 5x (nowadays 32mhz and 0mb plus overhead). In this case, notice the 345MB.
Now if you have 3 hosts, each would have 78. The idea here is one of them would go down. That would leave us with 156 (78 x 2). If you subtract 6 (the running vms), that leaves you 150.