I am not aware of a way to launch N instances of the node by passing N as a roslaunch parameter, however you can follow the approach given here : put the section of the roslaunch file that you want to replicate inside a namespace ns tag, and pass the value of this namespace as a command line argument. Then you can have a shell script that calls your roslaunch command (e.g roslaunch youbot_gazebo youbot_gazebo ns:=/robot\$i ) in a loop, each time with a different namespace value.