Adjust the quantity input values

Set the starting value, maximum value, minimum value, and increment amount for quantity input fields on product pages.

if ( ! function_exists( 'YOUR_PREFIX_woocommerce_quantity_input_args' ) ) {
	/**
	 * Adjust the quantity input values for simple products
	 */
	function YOUR_PREFIX_woocommerce_quantity_input_args( $args, $product ) {
		// Only affect the starting value on product pages, not the cart
		if ( is_singular( 'product' ) ) {
			$args['input_value'] = 4;
		}

		$args['max_value'] 	= 10; // Maximum value
		$args['min_value'] 	= 2;  // Minimum value
		$args['step'] 		= 2;  // Quantity steps

		return $args;
	}

	add_filter( 'woocommerce_quantity_input_args', 'YOUR_PREFIX_woocommerce_quantity_input_args', 10, 2 );
}

if ( ! function_exists( 'YOUR_PREFIX_woocommerce_available_variation' ) ) {
	/**
	 * Adjust the quantity input values for variations
	 */
	function YOUR_PREFIX_woocommerce_available_variation( $args ) {
		$args['max_qty'] = 20; // Maximum value (variations)
		$args['min_qty'] = 2;  // Minimum value (variations)

		// Note: the starting value and step for variations is controlled
		// from the 'woocommerce_quantity_input_args' filter shown above for
    // simple products

		return $args;
	}

	add_filter( 'woocommerce_available_variation', 'YOUR_PREFIX_woocommerce_available_variation' );
}

If you are looking for a little more power, check out our Min/Max Quantities extension!