I know it is a technical term thing(not overly important), but 30 feet by 18 inches I would consider to be a border and not a bed. Keep in mind I am not by any means a landscape person, just something that mind help you find some info in searches.
I know it is possible that some of the daylilies you get might not bloom this season, but in your location I think it very possible that most if not all of them would, depending on the vendor and the variety and the weather of course.
But at only 18 inches wide I think your 20 plants will be more than enough to fill up your bed. They will look very sparse maybe the first month or so but when the weather warms up they will fill in the bed pretty fast. Check the heights of the plants you have ordered, that can sometimes be a clue as to how long the leaves will be on the plant. Of course the short plants will often multiply more rapidly and could actually end up taking up more space than some of the taller plants in a couple of years. I know Primal Scream multiplies pretty slowly here, and the small 'Prairie Blue Eyes' and 'Crimson Pirate' multiply fast here.
Still, I think about twenty daylilies of any kind would fill you bed pretty darn full in a year's time.
