When you choose the Engineer as your soldier class, you are given the RPG as a gadget. However, unlike other gadgets such as the Anti-Vehicle Mine, the RPG can be resupplied using an Ammo Box. When the player starts the game, he has a default of 2 rockets. As the player levels though, he may use training points in order to upgrade the amount of rockets given. Just go to the Training List and select the "Increase RPG Ammo" option on the left panel called "Equipment Expertise". The first upgrade increases the rocket capacity to 4, while the second maxes it out at 6.