Apple today released Mac OS X 10.7 ... and you are good to install it on all 10 of those. Below is the actual section from the SLA relevant to this question: 2B(i) to download, install, use and run ...

Apple software that adds server functionality to any Mac machine running a current version of macOS. Called "OS X Server" when it debuted with OS X Version 10.7 (Lion), macOS Server adds ...