I’m bad at physics and worse at cooking. Do I need to leave ten chicken nuggets in the oven longer than if I only had one?
Probably not. For a given type of food, cooking time is determined primarily by the thickness of the piece of food. You have to cook it long enough for the heat to penetrate all the way to the center, and the more distance the heat has to travel from the outer surface, the longer that will take. So as long as your ten chicken nuggets aren’t all stuck together in a big wad on the tray (and are far enough apart that air can circulate freely around them), they’ll all warm up in the same amount of time that a single nugget would take.
This argument does ignore myriad small effects, like the dip in the oven air temperature due to adding the food, the effects on air circulation due to using a larger tray, etc. However, I’m pretty sure that to these won’t make much difference to within the timing precision of most amateur chefs.
It depends on how the food is layed out in the oven.
One chix nugget on a cookie sheet will take the same time to cook as 20 on the same sheet - assuming they are spread out. There is a very slight difference as the 20 frozen nuggets affect the temperature throughout the oven, but in a heat based oven, this would be about 20-30 seconds over ten minutes and the effect lessens over time.
Now, if you jumble them together, you cut off some points of interaction between the hot air and the frozen insides. It will take longer for them to cook.
Also, cookie sheets on multiple racks will affect the cooking time. The lower cookie sheet will absorb more heat into the metal - meaning at first, the lower cookie sheet will cook faster, but at the temperature equalizes throughout the oven this effect will have less effect.
(Cookies on a lower sheet will have darker bottoms than cookies on the higher sheet) - assuming heat element is below.
For top and bottom elements, two cookie sheets will heat differently.
That should provide enough basis for your understanding.
If you surrond a frozen pizza (24 min at 400°F) with 100 pounds of frozen rib roast, it will indeed take more than 24 min to cook the pizza, but that’s because that much frozen stuff can absorb a lot of heat from its environment. Before pizza cooking can commence at a normal rate, the oven must put out enough BTU’s to overcome the insulating capacity of the rib roast. It might take hours for that to happen.
Under more reasonable conditions, where the oven puts out enough heat to maintain its set point, cooking proceeds normally regardless of the quantity of food in the oven.
Alright, thanks for all responses 
That makes sense.
I wonder why, then, my three pot pies, a calzone and some gyozas (I was hungry, what can I say?) came out slightly cold in the middle while my one pot pie at the same settings the previous day turned out ok?
Was that enough food to absorb that much heat from the oven? They were individually small and thin enough as to not obstruct airflow, but I suppose some of 'em were frozen…
The trick with multiple items in the oven is to make sure they are spread out so that there is sufficient air flow between. Also, rotating pans (if they are in multiple pans) halfway through cooking times helps. Few ovens have 100% even temps throughout the cavity.
Keeping items separate will also help with browning. The same applies to cooking things on a skillet on the stove.
And while we’re on the topic, allow me to illustrate just how bad of a cook I am. Look at these [del]falafels[/del] charbroiled teriyaki knuckle-poop monster-things.
I got lazy by the third ball and gave up by the fourth. Potluck, anyone?
You may consider a new oven in the future, once which has a fan that circulates the air. They call them convection ovens, but obviously the principle at work isn’t convection.