Modern MacOS Server Restrictions
It’s a one-time payment, and it’s standard App Store licensing — you can use it on all Macs you own for personal use which also means if you use the same AppleID to download to 15 to 30 Macs, you might get an email from Apple or just be in violation of the Mac App Store terms. 

Overall, the server app enables command line and guidance management of many server features already present in MacOS, since it’s Unix, and adds a bunch more custom code and server functionality from the app that packages scripts and binary tools as opposed to a new replacement OS.