Depends on the time of year. Ive cruise Mid October twice, the first time in 2006, there weren't all that many teens, the second time in 2007 there were about 125 teens total on the ship if i remember correctly, and about half of those never came to the stack, and the rest broke down into groups, mainly by age.
My most recent cruise, spring break march 15-22nd there were 225 teens on the ship, and 127 in the stack on the first night for the 411, and about 50-75 at the dances each night, so a pretty good turn out.
It seems that groups of older teens tend to run about 10 people or so, while the younger ones(13-14, sometimes 15) tend to stay in larger groups and then have sub groups within the groups. This is just my personal observation on the 3 cruises i have done.
Also be warned that this summer 13-14's will be in Ocean Quest and no longer allowed in the Stack based on what multiple CMs in the Stack told me a few weeks ago