The muon densities at 450m from the core for showers above $10^{17.5}$eV and zenith angle between $0^\circ$ and $45^\circ$ are presented. Results are based on 1 year of calibrated data collected by the engineering array of the Auger Muons and Infill for Ground Array (AMIGA) detector. Taking into account systematic uncertainties as well as attenuation effects, the observations suggest that the current hadronic interaction models fail in reproducing the measured number of muons with energies ${>}1$GeV. Simulations at $10^{17.5}$eV and $10^{18.0}$eV show that for EPOS-LHC an increase of 38% is required at both energies, while for QGSJET an increment of 50% and 53% is needed at respective energies. Data have been combined with previous results on muon densities at higher energies showing the match in the evolution of the composition derived with the measurements of depth of the maximum development of the showers in the atmosphere ($X_\text{max}$). The current AMIGA observations show that the variation of the primary masses has no sudden changes in the
energy range $10^{17.5}$ to $10^{18.0}$eV.