Have in mind that when counting capacity in cpu cores, you must always count pCPU and not the logical CPUs. So for you this means 8 Cores available pr server x 4 (best practice should be around 4:1) = 32 vCPUs pr server.
This is however not a hard number, and must be adjusted according to usage and a pr enviroment experience. However my own experience is that the best practice recommendations from third party tests with 4:1 and 6:1 is a safe choice.