According to Wikipedia, TNEH has 2 exhibition floors, each is about 23000 square meter.
Now with Chinese Wikipedia it says TNEH's meeting rooms totals to 1860 square meter, which will fit approximately 1600 people.
So do the math, the exhibition floor will fit almost 20000 people. But if you subtract the space they will use for stages, chairs, etc... you are looking into probably 12000 people.
Now each exhibition floors is divided into 3 sections, roughly about 7000 square meter each.
Therefore you will get about 4000 people per section.
Conclusion - The concert will probably have 4000+ to 12000+ wotas in it, depending on wheather UFA will use 1 section or the whole floor!

lol... I'm all guessing though
Also on a side note, the will use the 4th floor as the concert hall because the 1st floor exhibition floor has support beams all over the area, whereas the 4th floor don't.
(Here is the website for TNEH, you can see their floor plan there)
http://www.twtcnangang.com.tw/?Lang=zh-TW