To display quantity input fields for simple products within your shop archive pages, use the following code.

/**
 * Override loop template and show quantities next to add to cart buttons
 */
add_filter( 'woocommerce_loop_add_to_cart_link', 'quantity_inputs_for_woocommerce_loop_add_to_cart_link', 10, 2 );
function quantity_inputs_for_woocommerce_loop_add_to_cart_link( $html, $product ) {
	if ( $product && $product->is_type( 'simple' ) && $product->is_purchasable() && $product->is_in_stock() && ! $product->is_sold_individually() ) {
		$html = '<form action="' . esc_url( $product->add_to_cart_url() ) . '" class="cart" method="post" enctype="multipart/form-data">';
		$html .= woocommerce_quantity_input( array(), $product, false );
		$html .= '<button type="submit" class="button alt">' . esc_html( $product->add_to_cart_text() ) . '</button>';
		$html .= '</form>';
	}
	return $html;
}

Add this code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Dessky Snippets plugin. Please don’t add custom code directly to your parent theme’s functions.php file as this will be overwritten completely when you update the theme.