Various drone applications have been deployed in real-world operations in recent years. As the use of drones increases in the future, unmanned traffic in cities will grow. There is a need to study the behavior of a large number of drones in shared airspace and understand the limits, restrictions, and capacity of the drone airspace. The limits and restrictions related to drone operations differ from manned aviation. In this chapter, various views on drone operations are summarized. The chapter aims at the assessment of airspace capacity using fast-time simulations. It focuses on the organization of the airspace to fit in a maximum number of flights with respect to the environment and restrictions. Metrics and their relationship to capacity are discussed, including various types of restrictions and how they impact the capacity. Simple scenarios are used to demonstrate elementary principles. Complex scenarios modeling future traffic over real-world cities were designed.