The process of staff planning and rostering in ANSPs has a considerable effect on ATCO productivity and, consequently, on capacity and cost-efficiency delivered to airspace users. The aim of this paper is twofold. Firstly, to establish a relationship between ACC taskload (represented by daily capacity profiles), rostering flexibility in place and the number of ATCOs needed. And secondly - to explore how varying rostering practices applied in ACCs affect spare capacity in the system. To that end, an initial mathematical model for optimizing ATCO staffing requirements has been proposed and tested on a case study involving three ACCs, reflecting different hourly variability in demand. The results suggest that - for different combinations of shift patterns and capacity profiles - substantial variation exists in both the minimum number of ATCOs (needed) and the associated spare ACC capacity. It has also been shown that a certain amount of spare capacity is inevitably present in the system due to inability to perfectly match ATCO hours with demand. Different options and enablers for exploiting such capacity are also discussed in the paper.