Recently, the properties of screening phenomena in concentrated electrolytes have attracted considerable attention. Molecular dynamics (MD) simulations were performed on an aqueous NaCl solution, and the cluster-size distribution of cations and anions was clarified. The cluster-size distribution exhibited approximately power-law decay behavior with a sufficiently high concentration. In addition, the fraction of isolated ions without pairing or clustering was also investigated. In concentrated electrolytes, the number of isolated ions does not significantly depend on the electrolyte concentration. This study clarifies the statistical physics of the cations and anions in concentrated electrolytes.