There is no law or upper/lower limit for the double layer cap. But just think about it for a min: the double layer is a thin layer whose thickness depend on mostly the electrolyte properties such as concentration, ionic species and experimental conditions such as voltage, frequency, scan rate (in the case of CV). There is only so much charge that can be stored in that region. DL cap also changes by roughness. And you also need to take into account that the region so called DL consists of three regions (compact layer, diffuse layer and space charge) following the pioneering works of Gouy-Chapman and Stern. So you might have to modify you eq circuit model to incorporate for those.

Good luck

Thank you Sir.

the double layer capacitance changes also with the metal surface, for high carbon steel and cast iron surface, the DL is much high