WooCommerce: Validate cart quantity.

functions.php

<?php
/**
 * Validate product quantity when added to cart.
 */
function cs_add_to_cart_validate_quantity( $valid, $product_id, $quantity ) {
    global $woocommerce;

    $valid = true;

    // Test quantity.
    if ( 50 > $quantity ) {

        // Sets error message.
        $woocommerce->add_error( __( 'A quantidade m&iacute;nima &eacute; de 50!' ) );
        $woocommerce->set_messages();

        $valid = false;
    }

    return $valid;
}

add_filter( 'woocommerce_add_to_cart_validation', 'cs_add_to_cart_validate_quantity', 10, 3 );

/**
 * Validate product quantity on cart update.
 */
function cs_update_validate_quantity( $valid, $cart_item_key, $values, $quantity ) {
    global $woocommerce;

    $min_quantity = 50;

    $valid = true;

    // Test quantity.
    if ( $min_quantity > $quantity ) {

        // Sets error message.
        $woocommerce->add_error( sprintf( __( 'A quantidade m&iacute;nima &eacute; de %s!' ), $min_quantity ) );
        $woocommerce->set_messages();
        $woocommerce->cart->set_quantity( $cart_item_key, $min_quantity );

        $valid = false;
    }

    return $valid;
}

add_filter( 'woocommerce_update_cart_validation', 'cs_update_validate_quantity', 1, 4 );

Opa! Muito bom estava mesmo atrás desta validação. Valew Cláudio!