For those who have multiple dogs, how many do you exercise at once?
Most of their exercise is running and playing with each other. They mostly play two-on-two (Nikon and Coke or Nikon and Kenya) but when I get home from work and they have a ton of energy, they play all together. If someone goes jogging, it's just one dog. If we go to the dog beach we take one at a time.
Do you have to split your group up depending on the size/breed/temperament/etc?
Nope, they co-exist fine.
Do you have to do anything special to exercise certain dogs?
Yes, Nikon requires more exercise than the other two combined. Now that he is old enough (physically) he has started jogging with DH. We also do a lot of fetch/retrieving.
Do you use any special collars/halters/harnessses when you exercise your dogs (no flaming please!! Just for interest's sake)?
Playing and fetch can be done naked. I used to bike with Kenya and she wore a Ruffwear harness. When Nikon starts (when I have a bike!) he will too. Kenya wears a Martingale if she jogs with DH, just in case she gets spooked when I'm not there. If Nikon is dock-diving or retrieving from shore he wears a prong with a tab and a long thin ASAT line (with no loops, handle, hardware, etc to get caught). The long line is mainly b/c the dogs have to be "on lead" at the dog beach, or in case he sees something and tries to bolt. It's a special line that does not get caught or tangled. Nikon wears a prong (on the dead ring) when jogging with DH and the lead is also attached to a Fursaver in case the prong opened. Nikon's starting agility this spring and will probably wear a Puppia harness, at the recommendation from a very good agility trainer/competitor who competes with high drive GSDs and uses the same types of motivation and reward that I will (crazy fast dogs running for toys and tug, we want drive drive drive and don't want agility to be an obedience exercise so we'd rather have the dogs pulling on a simple, very close fitting harness than a collar on their neck).
Do you do mostly on-leash, mostly off-leash, mixture?
95% off. The jogging with DH is really the only thing besides running wild with each other and playing fetch that we do that isn't seasonal like dock-diving and biking. Walks are laughable for my dogs, they just get them aroused and they come home even more hyper than before! Hard running, chasing, fetching, and wrestling are really what they want and need. I rarely walk my dogs because it's an impractical use of time. They'd rather go to the baseball fields and have a hard run.

*Kenya* UCH Alta-Tollhaus-Krieger Lamb Chop OB1 FO RA CL1-R CL1-F HIT TT TDI CGC
*Coke* VPC's Coca-Cola HIT CGC
*Nikon* SG UCH ptd Alta-Tollhaus Bono HIT CGC
*Ana* Alta-Tollhaus Havanna