You would have to implement this yourself by adding this functionality to the widget. Ideally you would then convince the Widget's developer to copy your change into a new version of the widget.
You could achieve this with the current version of the widget by validating the number of selected items in your onchange microflow and then using the validation feedback activity to report a message back to the user. Only possible downside with this approach is that you would have to run the validation check again on page submit.
Restricting the number of selectable objects would require a code change... if you want to contribute to the project with this enhancement I'd be more than happy to incorporate that into a new release.