The wiki page says the drain understanding part of knowledge domain should grant +4 or +1*Cleric Level to WIS, INT, and CHA, whichever is lower based on the number of enemies drained. It seems to give only +1 no matter how many enemies are drained or what level the cleric is. I've had a good few chances to test it.
Knowledge Domain Description (https://wiki.efupw.com/index.php?title=Turn_Undead#Knowledge_Domain)