It's really a guideline. Here's the deal... the number of models don't matter at all. What really matters is polygons. Every time your gpu has to render a polygon, it takes a bit of time. Now let's say you've got a building with 100 polygons. And you've got a box, which is 12 polygons.
So if I place 35 buildings, clearly that will require a lot more gpu power than 35 boxes. Now, in the monastery case, I used a lot of boxes (the roads and the water channels and the hedge walls). I would hazard a guess that the two buildings (two models) probably use more polygons than all the other models combined. The roads also have a reasonable LOD fade level (which is how far you can zoom out the camera before they disappear), while things like trees and buildings will stay on screen even at maximum distance (bad).
So in reality, it depends on a few factors (like opacity, polygons and LOD fade level). But in practice, it's easier to say to the rookie modder to use no more than 35 models, because then we can be sure that he's not going to place 35 buildings, and his polygon count will be reasonable.
@GaelicVigil not as far as I know. I usually take a look at the kingdom tiles in the tile editor before I start, though.