For installation and setup of PX4 SITL, ROS and MAVROS follow this link.
You can clone this package in your PX4 development workspace.
git clone https://github.com/Shobuj-Paul/nokia-formation-project.git
Run the simulation using the following command:
roslaunch nokia_simulation multi_drone_sim.launch
This launches four drones in a Gazebo environment. The launch file can be modified as per the comments given in the file to add or remove drones. You should get a scene like the following: