The Sentinel is running Windows Server 2008 R2 under the hood. So the lawyers at MS require it to have limits to be sold so cheap. There is no way to upgrade or “bump” the number of users. If you are just using it as a NAS box to have stuff available User Guest only counts as one.
25 desktops is the practical backup limit as well. If you need to backup more than 25 PC’s there is no limit to the number of Sentinels you can get. Just give them different server names.
If you have more than 25 users that need permissioned file access, you will need to look at full Windows Server 2008 R2 on different hardware.
