From what I've observed, I only have 4. Two males and two females. And they all take turns chasing each other off. That's not counting the bees and wasps that chase them all off.HaHa, 1 feeder gets six to eight cups a day now and the other gets two cups every three or four days. The one that uses less seems to have a really ornery bully wanting it all to himself, he patrols it and chases about all others away.
Also, never really noticed before but I have one that seems to whistle when it comes zooming by, kind of like a turbocharger whine.