Reconstructing the primary masses of ultra high-energetic cosmic rays (UHECRs) on event level can give insight into a manifold of open questions in astrophysics, for instance identifying individual sources or source regions. A reconstruction based on Air Shower Universality allows the determination of the relative muon content, Rµ, and the atmospheric depth of the shower maximum, Xmax, of an extensive air shower with data collected only from a surface detector. Both these observables can be linked to the mass of the cosmic ray. The upgraded surface detector of the Pierre Auger Observatory, AugerPrime, will allow an accurate reconstruction of these mass sensitive observables on an event level. We present the principles of Air Shower Universality as well as first results based on simulations to reconstruct the relative muon content of extensive air showers produced by ultra high-energetic cosmic rays using AugerPrime.