Based on the screenshot provided I am not sure whether you are referring to the MultiSelect widget or the DropDownTree widget.
For the MultiSelect you can set the tagMode configuration option to single to render only one tag with the number of selected items. As for adding checkboxes to the MultiSelect widget, you could refer to the HowTo article on creating a custom checkboxes template.
The DropDownTree itself provides a configuration option for enabling checkboxes as well as tagMode configuration. This example demonstrates a DropDownTree with checkboxes and a single tag displayed on selection.
I hope you will find the above information helpful.
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