I have set up about 625 on 3/16 this year and am very impressed. I have as many as about 40 taps on a few lines. Interesting to note that if there is a sizable leak the bottom taps will have pressure and will squirt sap out the tap when pulled out. When i fixed the leak the same taps that squirted out sap were then pulling vacuum. I am planning on setting up my whole operation on the 3/16 in the coming years. The slope does not have to be steep I have 35 on one line that is probably only on a 4 percent grade it would appear it siphons the sap out.
I have set up an experiment as well and found that 35 taps on 3/16 running into a 5/16 line with an additional 25 taps will not get the taps on 5/16 running that well on marginal days but it will make them draw a little vacuum about 6 inches on good flows.It would appear that 55 taps running into a 5/16 does not fill the line with a solid enough column of sap to not let air back unless a very good flow is present.