Thank you Chagan!! I should have known that anyone who is enough of a geek to understand theromodynamics would be totally unable to speak english about it!
Let me try another way to explain it:
There are three variables that come into play when cooking meat (or anything else) to a selected internal temperature using a heat source (that is, NOT a microwave). The variables are amount of heat supplied (cooking temperature), thickness of meat, and time. Changing one affects the others. So, if you lower the cooking temperature, you must either increase the time or reduce the thickness of the meat to compensate.
If it takes 1 hour to cook a 3 1/2 lb chicken to 175 degrees internal at 350 degrees, it will take the same amount of time to cook 2, 3,...n chickens AS LONG AS the temperature is maintained at 350. In practice, the number that can be cooked successfully is limited by the size of the cooking unit. You will at some point have enough stuff in the unit that air circulation is restricted sufficiently that nothing cooks properly. For all but the smallest units, that number is greater than two.
There, Chagan, I did it without once mentioning spherical chickens.