Count the number of rows in a Gravity Forms list field and store the result in another field.

ListFieldRowTotal

<script>
function ListFieldRowCount( listField, totalField ) {
    var totalRows = jQuery( listField ).find('table.gfield_list tbody tr').length;
    jQuery( totalField ).val( totalRows ).change();
}
function ListFieldRowTotal( formId, fieldId, totalFieldId ) {
    var listField = '#field_' + formId + '_' + fieldId;
    var totalField = '#input_' + formId + '_' + totalFieldId;
    ListFieldRowCount( listField, totalField );
    jQuery( listField ).on( 'click', '.add_list_item', function() {
        ListFieldRowCount( listField, totalField );
        jQuery( listField + ' .delete_list_item' ).removeProp( 'onclick' );
    });
    jQuery( listField ).on( 'click', '.delete_list_item', function() {
        gformDeleteListItem( this, 0 );
        ListFieldRowCount( listField, totalField );
    });
}
ListFieldRowTotal( 158, 1, 7 );
//ListFieldColumnTotal( form id, list field id, result - field id );
// enable dynamic population on field that is to hold result
</script>