The Zephyr drones they used are custom-made, largely from hobby parts, and cost about $2000 each. Getting so many into the air at once was challenging because they cannot be hand-launched like some smaller drones
The number of drones launched was more than just a stunt. Launching physically presents an opportunity to test out drone swarm behavior beyond simulations. There's no better way to find out how a drone swarm might act and control than to actually have a drone swarm.
The long-term goal is to have the swarms determine how to act on their own, and ARSENL reportedly intends to test this by eventually having a 50 vs. 50 drone swarm dogfight.
They only used one launcher. They can use multiple launchers. At present, the pre-flight safety checks limit launches to one every 30 seconds, but Jones thinks that they should be able to cut that to 10 seconds or less. He says that they may also build a second launcher, as there is no reason why the swarm has to be launched one at a time.
Once in the air, the drones communicated with each other via a system that uses high-powered Wi-Fi rather than conventional drone-communication systems, which would be swamped by the overlapping signals. The launch also gave an opportunity to test swarming algorithms with real drones rather than simulations.
SOURCES - New Scientist, Youtube, Popular Mechanics