There should be four drain holes, two in the front, and two at the back.
Having to clear them out regularily is not normal. Maybe once or twice a year is normal.
What comes out when you clear them?
You can remove the air duct covers near the rear wheels, to look down the cills, and see what is in there. Might be worth flushing them out if full of dirt.
Could be a problem if the contamination is rust, as cills can rust from inside out!