A number density of convective vortices (a number of vortices per km(2)) at the InSight landing site on Mars is estimated as a metric of vortex activity, using a theoretical expression that relates this number density to the recorded number of nearby passages of vortices experienced by the InSight Mars lander. A very high estimate, i.e. 56 vortices per km(2), of the average, over the whole period of observed vortex activity, number density of the vortices was obtained. Taking into account all the uncertainties involved in this quantity estimate, the latter can be reduced by a factor of about two, at most, but still remains at an unprecedented high level. For the sake of comparison, this methodology also applies to the Mars Science Laboratory and the Phoenix Lander vortices.