Double-atom catalysts (DACs) for harmful nitrate (NO3−) electroreduction to valuable ammonia (eNO3RR) is attractive for both environmental remediation and energy transformation. However, the limited metal loading in most DACs largely hinders their applications in practical catalytic applications. Therefore, exploring ultrahigh-density (UHD) DACs with abundant active metal centers and excellent eNO3RR activity is highly desired under the site-distance effect. Herein, starting from the experimental M2N6 motif deposited on graphene, we firstly screened the low-density (LD) Mn2N6 and Fe2N6 DACs with high eNO3RR activity and then established an appropriate activity descriptor for the LD–DAC system. By utilizing this descriptor, the corresponding Mn2N6 and Fe2N6 UHD–DACs with dynamic, thermal, thermodynamic, and electrochemical stabilities, are identified to locate at the peak of activity volcano, exhibiting rather-low limiting potentials of −0.25 and −0.38 V, respectively. Further analysis in term of spin state and orbital interaction, confirms that the electronic state effect similar to that of LD–DACs enable the excellent eNO3RR activity to be maintained in the UHD–DACs. These findings highlight the promising application of Mn2N6 and Fe2N6 UHD–DACs in nitrate electroreduction for NH3 production and provide impetus for further experimental exploration of ultrahigh-density DACs based on their intrinsic electronic states.