Rent a motorcycle and ride backwards. Just kidding. I am going to be helping a friend of mine shoot a mini-marathon (13.1 miles). There will be 15 of us (not really sure why he wants 15, but he's the boss) He is the promoter, so he wants pics of everyone (most at least) in the race. He is a good friend of mine, so I'm not getting paid. Our agreement is that if anything sells, then I get the $$$ from the sell of the photos.
Is your friend the promoter or a runner? If he is a runner, then I would think that 2 will suffice. You can get shots of him on the outbound leg and the return leg of the race. You'll need someone at the finish line to get shots of him/her when they finish. If you are taking promotion shots of the race, then you'll want a couple more. Get input from race promotion to see what they want.
Better yet, rig up a mount and run in the race. That would be a sight running down the street, huh?