I've "maxed out" my American Harvester (12 trays), so to have even drying time (so the bottom trays aren't finished hours before the top tray, I still have to rotate the trays if I'm using them all. I've been tempted to exceed the recommended 12 tray capacity at times (because even without rotating the trays, the top tray still dries MUCH faster than my old fanless Ronco dehydrator).
I think for most foods it would be safe to exceed max capacity IF you were frequently rotating the trays (I usually rotate the trays, swithching the top half of trays for the bottom about every 4 to 6 hours). However, since I usually am dehydrating jerky, I don't want to mess around with the recommendations too much, because meat bacteria issues are usually more risky than most veggie bacteria issues.
That being said, the only bacteria problems I ever had were with veggies. In Illinois, and without a fan, I could not dehydrate dense, wet fruits and veggies (such as grapes and tomatoes). The grapes never really dried enough to call raisins (and they had a weird fermented wine-smell), and the tomatoes were down right scary. A white foamy liquid formed, and it was hard to tell whether that was normal, or some kind of bacterial growth, so I left them be for a while, then the smell went sour and it was obvious something was wrong with them.
The tomatoes were so icky, that I've never been tempted to try drying tomatoes again (even though many people have great success doing so).
I'd be less concerned in my current dehydrator because of the fan, but the horrible smell of the "bad batch" stuck with me to such a degree that I can't even stomach the thought of trying again.
Because of my experience, I'd recommend at least for the first trial, not doing more than four trays of tomatoes, grapes or other dense, wet foods and put them closest to the fan (usually the bottom). If that works great then next time add more trays if that works well.