The conducted studies showed that projects of analysis and assessment of emergencies at critical infrastructure facilities have been initiated in Ukraine and abroad. The purpose of such a system is the formation of data for the development of architecture, demonstration analytics and a prototype of decision support capabilities, taking into account the use of currently available data and analytical methodologies. Unmanned aerial vehicles (UAVs), which work with ground and air control points in emergencies, are planned as the basis for the construction of such a system. The subject of the study is methods of determining the rational number of UAV flotillas taking into account the assigned tasks and characteristics. Currently, there is no method for determining the rational number of a UAV flotilla, taking into account the reliability of the aircraft. An urgent scientific and technical task is the task of creating a rational number of the UAV flotilla, taking into account the reliability of the aircraft and the necessary quality of monitoring the situation in emergencies. The purpose of the article is to develop a method for determining the rational number of UAV flotillas taking into account the reliability of the aircraft and the necessary quality of monitoring the situation in emergencies. Research methods – provisions of risk theory, probability theory, combinatorics, mathematical apparatus of reliability theory, mathematical methods of optimization. Research results: an analysis of factors affecting the structure of the UAV grouping was carried out; the dependence of task performance on the probability of UAV failure under the influence of interfering factors was obtained; it is shown that in conditions with a low impact of interfering factors, the reliability of individual UAVs does not have a significant impact on the performance of assigned tasks. It has been established that with increasing influence of interfering factors, the probability of completing tasks depends on the reliability of the aircraft. Conclusions: The proposed method makes it possible to create a rational number of UAV flotillas taking into account the reliability of the aircraft and the necessary quality of monitoring the situation with a rational number of UAVs in emergencies.