I'd like to confirm that indeed with a Sitefinity 4.0 Standard Edition license you can deploy unlimited number of instances of Sitefinity on sub domains like
If each of these subdomains will hold a separate Sitefinity installation, connected to separate database, then indeed each instance of Sitefinity will have a separate count of concurrent users - in this particular example - 15.
If you have 3 instances of Sitefinity that use the same database, that means that you actually have the same Sitefinity website accessed through 3 subdomains.
It's like having 3 TV sets in your home all displaying the same channel - it doesn't really matter which TV you're in front of - you're seeing the same thing, because all of them display content from the same source.
Since the Sitefinity instances share the same data from the database, these instances will also keep track of logged in users on any of these sites and the concurrent users count here is 5.
A Sitefinity website keeps all its data in the database, . If you have several installations of Sitefinity use the same database ( mind you, use the same database, not just take data from some tables using providers), they will display the same content and as logged users data is also stored in the database, they will all use that data to count the users too.
Grisha 'Greg' Karanikolov
Sitefinity Solutions Consultant