Mike, others...

DTS-ES and THX-EX are 6.1 discrete for encoding. A 7.1 receiver takes #6 and sends it to two Back Surround speakers. Many people find that one speaker directly behind doesn't work well with the positioning of our ears, so splitting the signal off-axis a bit helps.

So 7.1 in official surround encoding formats is really 6.1 with 2 speakers for one of the channels. Not unlike using 2 subs for a large room.

But, of course, if I've built 7 channels of amps into my receiver (speaking as mfg, here), I'm definitely going to call it 7.1 even though no recorded format actually is that many channels.

And there are several DVDs with DTS-ES available. Several...not many...certainly not anything like dozens.