In an old post I was curious about whether it is possible there are too many photons bombarding atoms in the vapor cell, causing saturation. However, this doesn't seem to be too reasonable of an argument anymore because based on the attached rough calculations, there tends to be more Rb atoms by a few orders of magnitude. So, a better calculation probably deals with the absorption coefficient/lifetime of the transition.

Also, I have just now realized how the vapor cell actually works: there is some solid Rb and gaseous Rb (2-5 mg for Rb cells according to Thorlabs) at room temperature and when heated, more of the Rb evaporates so there is then a higher number density. So in these calculations it seems reasonable to use the vapor pressure since in this case Rb vapor is exerting pressure on the condensed phases.