7-10, I'm not sure exactly. After looking at the pictures, 8, at least 7 of which are in service (not sure about 21, that is a mystery to me).
28 is the highest numbered track.
I know that 17/18 is the first platform that the Acela uses. I think that there is one more platform (so 19/20) making 21-28 the lower level tracks.
You can see that if you look at this Bing map:
http://www.bing.com/maps/?v=2&cp=qgh2np8kgmdq&scene=24020447&lvl=1&sty=b
This picture was taken around 3:00 PM based on the trains that I can see. You can see the Capitol Limited (with 3 P42s) preparing to leave from track 16. It looks like 17 is vacant and then there are 3 Acela sets on 18, 19 and 20 (you can also see Beech Grove 10001 at the end of track 20 - it has the distinctive box that contains a camera and lights on the end closest to the station).
Then you go to the lower level (you can see an AEM-7 parked just to the right of H Street on what should be track 21). When I left WAS on the Cardinal, I'm pretty sure that we left from track 22, so if that one goes through I would think that 22-28 all go under the station. The only one I'm up in the air about is 21 (which is the one that you referred to in your OP). Hope this helps!
Edit 2: Picture was taken before 3:20, the long MARC set that's always used for the 3:20 (formerly the 3:24, 3:27 and 3:34) is still down there on 28 track.
Edit 3: Just above the MARC set on 28 you can see a LD consist - that'd be the the Silver Star changing power. Assuming that she's on time, that narrows it to between 2:45 and 3:15. Since you can see the sleepers on the rear, that limits the date range that the picture can be taken as well. How long were the sleepers running back there?