The fifth generation (5G) of mobile telecommunication network is designed with an ambition to be a network faster, stronger, better and smarter than its predecessor. With the digital transformation, all industry sectors will develop new applications with new requirements regarding telecommunication networks that 5G should be able to meet. To meet the requirement of future 5G use cases and applications, it is crucial to study the complexity of such network system by distinguishing different parts, layers, components as well as their interdependencies. This paper describes the 5G networks from an End-to-End perspective (device, radio network, core network, data network) and from a multi-layer perspective (orchestration, virtualisation/containerization and infrastructure) to show how this system (or system of systems) is complex, especially when we address resilience challenges. Resilience requirements and challenges are further explained by proposing relevant scenarios and use cases. In this paper, we mainly intend to highlight 5G network complexity and open a discussion on methodologies to model such complex network for its resilience study with the hope that this paper could inspire the future study of researchers in the related field.