Magento – How to limit product max quantity per customer

Take a look @ Magento Maximum Allowed Order Amount, you would have to create a custom module to add this feature.

Create an observer for sales_quote_save_before

<config>
    <frontend>
        <events>
            <sales_quote_save_before>
                <observers>
                    <inchoo_maxorderamount_enforceSingleOrderLimit>
                        <type>singleton</type>
                        <class>Inchoo_MaxOrderAmount/observer</class>
                        <method>enforceSingleOrderLimit</method>
                    </inchoo_maxorderamount_enforceSingleOrderLimit>
                </observers>
            </sales_quote_save_before>
        </events>
    </frontend>
</config>

In your observer

class Inchoo_MaxOrderAmount_Model_Observer
{
    private $_helper;
    public function __construct()
    {
        $this->_helper = Mage::helper('inchoo_maxorderamount');
    }
    /**
     * No single order can be placed over the amount of X
     */
    public function enforceSingleOrderLimit($observer)
    {
        if (!$this->_helper->isModuleEnabled()) {
            return;
        }

        //todo - look up customer info and check if they already purchase this item
        $quote = $observer->getEvent()->getQuote();
        if ($quote->getCart()->getItemsCount() == 1) {

            Mage::getSingleton('checkout/session')->addError('limit only one product per order');
            Mage::app()->getFrontController()->getResponse()->setRedirect(Mage::getUrl('checkout/cart'));
            Mage::app()->getResponse()->sendResponse();
            exit;
        }
    }
}

       -----------Create controller-------------

         <?php
class Magento_Guys_IndexController extends Mage_Core_Controller_Front_Action
{
    public function indexAction()
    {
        echo "Thank you !";
    }
     public function genCartAction()
    {
        $id = $this->getRequest()->getParam('pid');
        $_product = Mage::getModel('catalog/product')->load($id);
        $buy = Mage::helper('checkout/cart')->getAddUrl($_product);
        echo $qty = (int) Mage::getModel('cataloginventory/stock_item')->loadByProduct($_product)->getQty();

        //echo $id;
        //echo $this->getRequest()->getParam('id');;
    }
}
?>

    ----------------Change Add to cart code-------------------

    <?php if ($_product->isAvailable()): ?>
              <b class="available_quanity" style="display: none">Available Quantity :</b> <span id="simplestock" class="simplestock">Please select a color to view the quantity</span>
            <?php endif; ?>
            </div>
            <?php if(!$_product->isGrouped()): ?>
            <div class="qty">
                <label for="qty"><?php echo $this->__('Qty:') ?></label>
                  <?php $i = 0; ?>
                  <select id="qty" class="input-text" name="qty" style="width:50px;">
                    <?php while($i<=(int) Mage::getModel('cataloginventory/stock_item')->loadByProduct($_product)->getQty()):?>
                      <option value="<?php echo $i; ?>"><?php echo $i++; ?></option>
                    <?php endwhile; ?>
                  </select>
            </div>
            <?php endif; ?>

    ----------------Add Ajax for get Quantity---------------

    <script>
    jQuery(document).ready(function() {
        jQuery(".product-options select[id^='attribute']").on('change', function() {
            var id = getSimpleProductId();
            var qty = "";
            jQuery('.available_quanity').show();
            jQuery("#fancybox-loading").show();
            jQuery.ajax({      
                type: "POST",
                data: 'pid=' + id,
                url:'https://www.thewirelesscircle.com/guys/index/genCart',
                success:function(response){ 
                    if (response) {
                        qty = response;
                        var x = document.getElementById("qty");
                        var i;
                        removeOptions(x);
                        for(i=1;i<=qty;i++) {
                            var option = document.createElement("option");
                            option.text = i;
                            option.value = i;
                            x.add(option);
                        }
                    }
                    jQuery("#fancybox-loading").hide();
                }
            });

        });
    });

    </script>

    ---------------Get Selected Option Id in Magento-------------------

    <script type="text/javascript">
        function removeOptions(selectbox)
        {
            var i;
            for(i = selectbox.options.length - 1 ; i >= 0 ; i--)
            {
                selectbox.remove(i);
            }
        }
        function getSimpleProductId() {
            var productCandidates = [];
            jQuery.each(spConfig.settings, function (selectIndex, select) {
                var attributeId = select.id.replace('attribute', '');
                var selectedValue = select.options[select.selectedIndex].value;

                jQuery.each(spConfig.config.attributes[attributeId].options, function(optionIndex, option) {

                    if (option.id == selectedValue) {
                        var optionProducts = option.products;

                        if (productCandidates.length == 0) {
                            productCandidates = optionProducts;
                        } else {
                            var productIntersection = [];
                            jQuery.each(optionProducts, function (productIndex, productId) {
                                if (productCandidates.indexOf(productId) > -1) {
                                    productIntersection.push(productId);
                                }
                            });
                            productCandidates = productIntersection;
                        }
                    }
                });
            });
            return (productCandidates.length == 1) ? productCandidates[0] : null;
        }
    </script>