Use this snippet to set a default quantity for a component of a composite product.

wc-composite-products-default-component-quality.php

<?php
/*
Use this snippet to set a default quantity for a component of a composite product.

Before using this snippet, replace 1234 with the ID of the component for which you’d like to set a default quantity. 
You can find the ID of each component by: navigating to Product Data > Components and hovering over a specific component
*/

add_filter( 'woocommerce_composited_product_quantity', 'sw_wc_cp_set_default_quantity', 10, 6 );

function sw_wc_cp_set_default_quantity( $default_quantity, $quantity_min, $quantity_max, $product, $component_id, $composite_product ) {

	if ( $component_id === '1234' ) {
		$default_quantity = 5;
	}

	return $default_quantity;
}