The default memory allocation for a new macOS guest should be 4GB based on: macOS Catalina the current version of macOS system requirements specify 4GB RAM macOS Catalina can be installed from a Mac's recovery partition and Parallels Desktop will allocate 2GB RAM to this virtual machine. My own experience of configuring some macOS Servers suggests that when installing a Catalina guest the memory size needs to be 4GB, High Sierra can be 2GB but might struggle at that, and OS X Server Leopard will work fine with 1GB RAM and a single CPU core since it will constantly run at 100% on any cores it is allocated despite my repeatedly reporting this bug. Vmware Fusion configures guest machines differently for different versions of OS X and macOS because one size does not fit all. I think it is time that Parallels Desktop worked in a similar fashion and fixed issues with OS X Server Leopard virtual machines. One of the reasons admins virtualise is to keep legacy software supported with modern hardware so this is an important issue.