So I've been running multiple VMs (sometimes up to 5 or 6 at a time) on my 2012 Macbook Air with 8GB memory. Sure not the greatest performance, but it worked and made windows update and parallels tools updates easier. Since updating to PD12 I can no longer run more than one VM at as time. When first VM is open, all is fine. When second VM opens, one of them always freezes...usually the first VM opened. If I pause the running VM, the frozen VM starts to work. If I unpause the paused VM, one of them freezes. Note that one VM is older (built with parallels version 9 or 10) running Windows 7 and other is built with PD 12 running Windows 10. Both have latest and greatest parallels tools installed, run full screen, all of the sharing and integration stuff shut off.