How many channels do the DX series go up to? I know of the DX6 and DX7. Do they go higher?

No not yet but soon tower hobbys and horizon will start selling there own line of spread spectrum 2 ghz radio's and will offer a model with 10 channels and more range.

If you need more channels and already have a nice radio look at this:
http://www.xtremepowersystems.net/xtremelink.php

I do have enough channels. I have a Futaba 9c so I hope 9 channels will be enough for whatever I would like to use them for. A wireless camera maybe.