Ram requirements for VM's

Linux in general works best with at least 4GB of RAM, but it all depends on the workload. I assume you are using Kali Linux for penetration testing or tools. It seems a strange choice for anything else, although it is able to be used in other ways. Creative and development workloads will require more memory -- how much do those workloads require regardless of how you have installed the OS. The VM will require at least that much memory, plus you will need more memory for the host OS (in this case macOS). I really would not run macOS with less than 8GB RAM dedicated to macOS, so at a minimum you would need need at least 12GB of physical memory for the host just to use Linux with basic workload. Anything requiring memory I would assume should have at least 8GB of RAM for the VM, perhaps even more. Figure out how much memory those workloads need and that will determine how much memory you need to reserve for the VM.

Another consideration is will you be doing anything intensive or requiring lots of memory on macOS while you are using the operating systems within the VM at the same time? If so, then you will likely need more than 8GB of RAM dedicated just to macOS.

As much as you give it. With virtual machines RAM is user configurable.

For decent performance at least 8GB would be recommended if you have a computer with 16GB or more.