It was a “Large” Training Run:

Active duty and Air Force Reserve Command airmen on Dec. 21 flew the largest formation of C-17 airlifters from a single base to date, when they took 20 C-17s airborne from Charleston AFB, S.C. After taking off in 30-second intervals, the airmen from Charleston’s active 437th Airlift Wing and Reserve 315th AW completed more than 500 training events, including a massive airdrop. The training run enabled Charleston to certify seven aircrews as formation airdrop leads and test ground maintenance and aerial port airmen in the mechanics of the strategic airdrop, meant to deliver a brigade’s worth of soldiers and equipment within 30 minutes.