4GL Left list picking - number of selected items

Hi folks

I'm looking for a possibility to get information about how many items are selected in a picking left list on action $PICKE and $DEPICK. My goal is to allow the user to only select one item and show an exception message if he tries to select another one.

Is there any standard method/object/property I can use to achieve this?

Thanks in advance, wish you guys a good weekend!

Cheers, Benjamin