Source code for https://formidable-masterminds.com/increment-repeater-field-row-counter/

increment_repeater_field_counter.php

<?php
add_filter('frm_setup_new_fields_vars', 'my_repeater_counter', 20, 3);
function my_repeater_counter($values, $field, $args ) {
	if ( $field->id == 234 ){
		$values['value'] = $args['key_pointer'] + 1;
	}

	return $values;
}

portable_increment_repeater_field_counter.php

<?php
add_filter('frm_setup_new_fields_vars', 'my_repeater_counter', 20, 3);
function my_repeater_counter($values, $field, $args ) {
	if (  $field->field_key == 'demo_name' ){
		$values['value'] = $args['key_pointer'] + 1;
	}

	return $values;
}