That’s easy. Each port can deliver up to 100W (20V@5A). No monitor should be pulling anywhere near that kind of wattage, especially if designed to be portable. I’m guessing around 10-15W max per monitor? Given the amazon answer from the manufacturer for this portable monitor it seems 10W is a reasonable guess. I would not go higher res or higher refresh rate as that will increase power draw.
That has some schematics if you want to know more technical stuff.
It looks like you wont be able to daisy chain monitors together unless they support PD passthrough and video on the same cable and that seems unlikely and would definitely drive up the cost of the monitor so You’ll need to use all ports to make this a reality but it is 
achievable. Your best bet is to find a portable monitor with an integrated USB hub to avoid any docks.