Cu is a unique metal that catalyzes carbon monoxide/carbon dioxide (CO/CO2) to form high-order hydrocarbons and oxygenates through the CO/CO2 reduction reaction (CO/CO2RR) at decent selectivity and productivity. While this has been shown, the limits of the system are still unknown, i.e., the minimum amount of Cu needed for the CO/CO2RR, and the maximum activity that trace amounts of Cu can achieve. Here, we have investigated the activity and selectivity of trace Cu with atomic dispersion over a range of loadings below 2 μg cm−2 and have quantified the mass activity/turnover frequency (TOF) of trace Cu catalysts. A Cu loading of at least 0.042 μg cm−2 initiates the CORR activity with 30% faradaic efficiency (FE) at a partial current density of 102 mA cm−2 forming predominantly CH4. The selectivity moves to C2-based products (CH3COO−, C2H4, and CH3CH2OH) with 70% FE as the Cu loading increases to 0.333 μg cm−2, and increasing the Cu loading to 0.812 μg cm−2 results in a 78% FE to C2 with CH3COO− accounting for 42% of this. The highest mass activity for CH4 reaches 2435 A mg−1 of Cu, corresponding to a TOF of 267 s−1, while C2 activity reaches 584 A mg−1 of Cu, leading to a TOF of 145 s−1. Both TOFs are several orders of magnitude higher than the reported values. Different from the CORR, the CO2RR demands a higher Cu loading and primarily generates C1 (e.g., CO and CH4). Metal impurities can be extended to others that are active towards the CO2RR, such as Zn for CO2-to-CO conversion. Thus, we suggest that the effect of trace metal impurities must be quantified when developing carbon-based metal-free and coordinated single-atom catalysts for the CO/CO2RR in order to avoid overestimating their activity.