using the following statements this will force a selection of at least one if these actions are added to
Settings-->Document Properties-->Field Event Trigger-->[Your Field] --> Edit Action--> Add--> Field
=if( GetSelectedCount([Your Field]) < 1,'Your Field')
--> Search String
=fieldvalue('Your Field' , FieldIndex('Your Field', Max([Your Field])))
This will select the max record #
This will jump to the max record if the user trys to unselect the only selected field selection
It will also select it if the global clear button is clicked on top of page.
However if you need to unselect the field upon leaving the TAB, you can - add an action to "clear all" upon leaving the tab and it will clear it.
I'm sure you can add another action to limit to 15. I do not know of a way to select 15 however if the user selects 16. however you could select 1 if the user trys to select 16.