Hi there;
I'm assuming that you've purchased the program fairly recently, as this is a very common "newbie" question. Considering that, I'm guessing you are using SL or SX? I believe both use the same manual. Check out p.625-626 of the manual, paying particular attention to muting the tracks you don't want included in your bounce down. (suggested in the italics under one of the steps there)
BTW.... why do you want to do this? If you simply want to control them all simultaneously (ie. EQ or FX or levels on the whole group like selecting seven drum tracks to compress, but not the three cymbal tracks....) you would be best off creating a group track and doing things that way. If it is to save processor power, consider applying the FX to the track, and then turning off FX processors. This can be undone later if you need to change it.
Cubase is not limited (like Pro Tools LE IS...) to a given number of tracks. If your computer can hack it, you could have as many tracks as you want.
Chris