The 20 will fit 12 babies with no problem. They only sit under there when they are tiny. As long as you have them in a room with an ambient temp of 70 or so, they won't spend much time under it after a couple of weeks. I have my chicks in a box with a 40 watt bulb over it for light and a little warmth. They are three weeks old, and they rarely go under the brooder any more. But if you can afford it and are planning to possibly do larger flocks, get the larger one.
I raised 10 day olds under my Ecoglow 20 and it worked just fine. When they started getting bigger I did have to adjust the height requirements. They grow so quickly though, and I remember thinking that if I had more, I would have wanted a bigger one. Remember they do huddle together so much, and help stay warm that way. Also, don't get frightened when you see the occasional chick laying under the Ecoglow with it's head sticking out and looking like he is passed away, sometimes it just happens. It used to make my heart stop!
Either way you can't go wrong, but if you think you might raise more in the future, just buy the larger one.