If you buy WordShell, then you have the right to use it to manage, backup, etc., unlimited WordPress sites. The websites you manage can be on as many machines as you like), forever. You can use the www.wordshell.net website to get unlimited support and upgrades (if you purchased an upgrade subscription). If you purchase a subscription package then it is per-machine where you install WordShell (not per-website – those are unlimited).
What do I get for my money?