For this, you should check system requirements for the OS and applications. However, you can rely on the perfect ratio. For instance, your host machine has 8 GB of memory while running Windows 10 or ESXi 7.0 with VMware Workstation (Windows Server 2019 VM). If you set 4 GB of RAM for a VM, 4 GB is left for your host OS. It is more than enough for decent functioning. Note: This example doesn't include additional applications that might consume significant resources.