The numbers are simply a way of keeping track of the progress of the software as it is being constantly developed, improved, and slightly changed as time goes bye.
The main version number, in this case 9, is most important, and if it were to change, then you'd know there would likely be significant changes. The last number, in this case 21 is a 'build number' to distinguish a different build (of the same version usually). See this page for more detail: https://en.wikipedia.org/wiki/Software_versioning
If you want to check the version, build etc you have, then you can see it from main apps interface by clicking cog wheel beside My Account, and look under About SecureAnywhere tab.
If I may just add here...I believe that you are referring to what you see in the online console (HERE) when yo look uder PC Security? If that is the case then please be advised that Webroot picks up the system name of each PC and uses that to identify the registered/protected device.
If running WIndows 10 (or any version of Windows for that matter) you can personalise the name of your PC...I would caution that before you do that you make sure that there is nothing that depends on that information for anything...unlike but in some cases it is used in the same way as WSA uses it.
To change this name (under WIndows 10) click on Settings > System > About and then you should see the System Name together with a button marked 'Rename PC'. If yo click on that per PC you can then change their designation and eventually the online console should reflect this change allow for easier identification.