A series of constant-temperature/constant-pressure molecular dynamics simulations of aqueous NaCl solutions at different salt concentrations is carried out to investigate the structure and the dynamical properties, The simulations were performed with the number of molecules ranging from 256 to 2000. The simulations cover several nanoseconds to ensure the convergence of the results and to enable a proper determination of ion-ion radial distribution functions. The flexible SPC water model is used as the solvent, while the ions are treated as charged Lennard-Jones particles, Only a weak influence of the salt concentration is found on the ion-ion pair correlation functions. The structures of the hydrated shells around ion pairs are studied using three-body correlation functions, The self-diffusion and interdiffusion coefficients are found to decrease with an increase of salt concentration. Molar conductivities are calculated al different salt concentrations. The residence times of water molecules in the hydration shells as well as the residence limes of contact and solvent-separated ion configurations are determined.