It is a viable question that is often asked, but the answer is not really easy nor straight forward. To be clear if you want this kind of realtime information you should be looking at a analyses tool like VMware vCenter Operations or Dell vOPS (formerly vKernel / Quest) these tools can (when you purchase the right licenses) give you the exact amount of VMs you could add to your environment while retaining the needed failover capacity, vOPS even has the ability to "play" what if scenarios and have the results on these before you actually do anything in real life to your environment.
I will try to write one (or more) blogs post in the near future after I have taken these tools for another test run.
In the mean time, the question remains. Or if your company can't or does not want to invest a considerable amount of money in one of these tools, and believe me they are pricey (but what they provide on useful information sure makes up for their price).
How many VMs can I add to my current environment keeping in account the needed (or requested) failover capacity ?
For me a alternate way of getting a idea on how many VMs I can add to a specific environment is by running a PowerCLI script that rounds up a average CPU and Memory use of the current VMs and you manually specify what the maximum CPU and Memory resource usage per cluster may be (Only for non HA clusters).
On a side-note; In theory you should be able to calculate how many VMs you could add by using the HA slot size , but in real life there are to many dependencies and variables for this to be useful especially if you don't use reservations on your VMs. Anyway I am not going to write to much about HA, DRS slot sizes, reservations.... because there a other (Dutch) people that can (and actually did) write books on these topics. A must read for anyone who want's to be a better VMware admin, consultant or architect is VMware vSphere 5.1 Clustering Deepdive by Duncan Epping and Frank Denneman.
So back to the quick, simple and somewhat educated guess way of getting a idea on how many VMs you can add to your environment. As said earlier I think the quickest way of getting such a result is by having a PowerCLI script get the average values of all VMs currently running and set the maximum resource usage allowed by yourself.
The script I use is fairly simple and quick to run, and will give you results that you can use as a sort of guideline when you get this question and you need to answer it.
By default the output is directly to the CLI, but you could also output to csv, xlsx or HTML if you want. The CLI output will look something like this.
Both clusters in the screenshot above have more then enough CPU resources, but when you look at the MEM resources they both are around 10% growth allowed based on the current load and failover level. These results are from HA enabled clusters, so no manual config needed. The script uses in this case the "EffectiveCpu", "EffectiveMemory", "FailoverLevel" and "NumEffectiveHosts"from "DasConfig" to calculate the results.