How can i count all my colonized planets in my empire and store it in a variable?

I tried:

<Property Name="ArgosPlanets" BaseValue="1"/>

<Modifier TargetProperty="ArgosPlanets" Operation="Force" Value="$(OwnedPlanets)" SearchValueFromPath="true" Path="../ClassEmpire/ClassColonizedStarSystem,ColonizedStarSystemStateColony/ClassColonizedPlanet"/>

and use it then 

<BinaryModifier TargetProperty="SystemGrowth" Operation="Addition" Left="10" BinaryOperation="Multiplication" Right="$(ArgosPlanets)" Path="./ClassColonizedStarSystem"/>

But this did not work. What iam doing wrong?

What is the scope, is this a faction trait?

Its a StarSystemImprovement.