The HDR window position can be monitored by looking at the window meter in the HDR tab of a bus. You can also assign a Game Parameter to its value. From there, you can map the Game Parameter to any property of any object via RTPC. For example, you could use the window position to tweak the maximum number of sound instances.
To attach a Game Parameter on the HDR window position:
Inspect the HDR bus to see its content in the Property Editor.
Click the HDR tab.
In the Window Top Output Game Parameter group box, click [...] to browse for a Game Parameter.
Set the range for which the Game Parameter will be set, in decibels on the HDR bus input.