There are several ways to do this:
$collection = Mage::getModel('...')
->getCollection()
->setPageSize(20)
->setCurPage(1);
Will get first 20 records.
Here is the alternative and maybe more readable way:
$collection = Mage::getModel('...')->getCollection();
$collection->getSelect()->limit(20);
This will call Zend Db limit. You can set offset as second parameter.
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