They'll fight over them no matter how many. I used to put out 10 and this year have 6 so I change them more often so the sugar water doesn't forment or mold. Used to use glass feeders and found them hard to keep clean, found that the cheapest $3.50 feeders work just as well and replace them when they get too hard to keep clean. So far am going through about 3 quarts a day trying to keep up with them. 
Holy cow, I have never seen so many hummingbirds!
They are really dark,what kind are they?
Your picture is a perfect illustration of what I meant. If you have one feeder an interloper would be instantly divebombed by the dominant bird, who keeps constant watch between their own feedings. With two so close it’s too hard to keep up so you have groups of birds feeding instead of fighting.
I’ve seen those little hummingbird feeders at the Dollar Tree!