You need to find out the output of the adapter and the current draw of each pedal. As long as the total current draw is less than the output of the adapter you can have as many pedals as you want on it. Some pedals draw very little current, others are pigs. My DD-20 is a pig.
Also consider how many pedals you will have on at any given time. I doubt you will have all twenty on at once.
Most pedals have a plate on the bottom that gives you current draw in mA. If you add all of them togather you will have max draw for all twenty, which may result in a PS that is larger than you really need.
Figure out your most used combinations and see what the draw is for each of these combos. If you take your highest combo draw and add 10-15% you should be OK
Most pedals have the same current draw wether they are on or off. In some circuits, the LED will draw a little extra, but that's only a mA or two, so the difference isn't something to calculate with. One exception is the Korg Pitchblack, which actually turns the power off in bypass mode, dropping the 35mA max current draw to 0.07mA. But as the other pedals use the same even when you're tuning, you'll still have to calculate with the max draw for the Pitchblack.
The PP2+ has 100mA for each output (250mA for outputs 5/6), so as long as you stay below 80-85mA for each output (having a safety margin is always good), you can power as many as will fit within that figure. For instance, a DD-3 draws 55mA, so it should have its own (100mA) output. But a DS-1 only draws 4mA, which will leave plenty of room for others.
You will lose the isolation between the pedals powered from the same output, so you may have to experiment with different combinations to find the pedals that play nice with each other.
No. I don't think I'll be running 20 pedals at once. I did see someone post (in the pedal board thread) pictures of his PT Pro with both a PP and DC Brick so I was wondering how to figure if one PP would suffice for me. Needless to say, I have not thought about power draw, etc for a couple of decades.
Why? Each output (except outputs 5 and 6 when they're set to the "L6" mode) is regulated, and should stay at 9vDC right up to their max current output. The L6 mode is unregulated, and puts out about 16 volts DC without load, dropping to about 10-11 volts under full load.