In his new book, The Last Centurion (out as an eARC), John Ringo has a good description of how much mass, in terms of supplies, a modern army uses. Think a football stadium. Actually a couple of football stadiums. Filled.
So how many vehicles are involved? Here's a good description (continued after the fold, due to one "not-kid-friendly" word):
With the attachment of 1st LAR Battalion, Second Tank Battalion, a combat service support company, a combat engineer company, and an assault amphibian company, Fifth Marines quickly transformed into Regimental Combat Team Five (RCT-5). It was a hell of a force, comprising approximately six thousand personnel and nearly two thousand vehicles of all types—M1A1 Abrams tanks, amphibious assault vehicles (AAVs, or "amtracks"), LAVs, hummvees (or HMMWVs—high mobility, multipurpose wheeled vehicles), seven-ton trucks, bulldozers, and fuel tankers. For division planning rehearsals, the commanding general had devised the idea of "Lego drills," in which each unit would bring a mat to which Lego blocks had been affixed to represent every vehicle in each RCT. Only that way could the commanders get an idea of the magnitude and scope of the operation ahead, as well as the space considerations involved in housing a regimental combat team's allotment of vehicles and Marines and allowing them room to maneuver. I saw the Lego set for RCT-5 one day while I was at their headquarters. It was staggering. As I stood there staring at it, one of the regimental operations officers came up to me.
"What do you think?" he asked, motioning to the thousands of tiny plastic icons arrayed before me. I nodded my head and looked back at him.
"That's a shit-ton of vehicles," I replied.
(The Highway War: A Marine Company Commander in Iraq, Maj. Seth W. B. Folsom, USMC)