Magento – Shopping Cart Price rule – tiered

At the conditions tab select

Total qty of products = 2 with following conditions:
Sku is one of a,b,c

At the actions tab select
Action: fixed amount for whole cart
Amount: x

For 2,4,8 create multiple rules.

Don't know other solution without extensions or custom development

$appliedRuleIds = Mage::getSingleton('checkout/session')->getQuote()->getAppliedRuleIds();

This will give you the ids of the rules applied to the quote separated by comma.
you can turn them into an array like this:

$appliedRuleIds = explode(',', $appliedRuleIds);

if you want to get the rules applied as objects you can do this:

//$rules = Mage::getModel('salesrule/rule')->getCollection()->addFieldToFilter('rule_id' => array('in' => $appliedRuleIds));
$rules =  Mage::getModel('salesrule/rule')->getCollection()->addFieldToFilter('rule_id' , array('in' => $appliedRuleIds));

foreach ($rules as $rule) {
    //do something with $rule
}