I suspect this sort of question is going to be increasingly common in the time to come.
the generic answer is something like this;
The "lines per page" is an expression.
The Clarion CHOOSE command let's you place an "IF" statement inside an expression.
the p_web.IsMobile() method tells you if it's mobile or not.
so, in your case, the "Lines per page" expression becomes something like
CHOOSE(p_web.IsMobile()>0,5,10)
Where 5 is the number of lines for mobile, and 10 is for desktop.
Also - do not be tempted to drop the >0 part of the expression - it's important.
cheers
Bruce