woocommerce/docs/code-snippets/adjust-quantity-input-values.md

Adjust the quantity input values

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

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 Code snippets plugin. Avoid adding custom code directly to your parent theme’s functions.php file, as this will be wiped entirely when you update the theme.

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!