Whether an infrastructure is correctly dimensioned and whether it needs to be expanded and if so, by how much physical capacity, depends on many individual details. In addition, the workload is very different in most infrastructures. Therefore, there is no blanket statement or a simple vCPU calculator. At the latest when CPU, memory or storage is used to 70% or more, you know for sure that you have to expand. But performance problems can occur much earlier.
If you look only from the perspective of CPU performance, you should pay attention to the physical core to vCPU ratio. As a best practice it has been proven that a pCore : vCPU ratio of 1:1 - 1:3 works well with most workloads and from 1:5 it becomes bad for some workloads. However, there are also infrastructures where 1:2 already leads to performance problems and infrastructures that hardly notice performance bottlenecks with 1:10.
So I would recommend you either do the math yourself and calculate the ratio and check the CPU Ready values of all hosts over a longer period of time or what might be easier for you: Use vRealize Operations Manager and show your boss the Capacity Dashboards.