It seems in order to change the qSelectionThreshold value you will have to change a user setting which is something not possible at the moment. They said this functionality it's on the roadmap, but this was more than a year ago...
Check below thread:
I correct myself, it seems is possible to extend this threshold in QLik Sense Enterprise as well.
You need to modify the setting.ini file usually located in C:\ProgramData\Qlik\Sense\Engine
Under [Settings 7] section add below line:
Remember to leave a blank line as the last line in the file otherwise the variable won't be captured
Restart the Qlik Sense Engine service and now you should be able to select up to 15 items.