Do I have to adjust oven cooking times from recipes for more / less food?
Many ovens do not have a thermostat. They have a timer, which turns the heating elements on and off in a predetermined pattern. As a result, the oven is a very imprecise heating instrument. It is frequently miscalibrated, and even for a calibrated oven, the temperature you set is the average temperature, while the actual temperature of the air inside varies in a roughly sinusoidal pattern, and the internal temperature of the baked good is even more irregular depending on its composition, location, choice of baking tin, oven type, etc. Even if the oven has a thermostat, it only (roughly) regulates the air temperature, not the internal temperature of the food. 

As a result, the change in heating speed caused by adding a little bit more mass is really insignificant when compared to the huge variability of heating speed which is inherently present between ovens. 

Luckily, many recipes are not very sensitive, and taste good no matter how much heat transfer actually happened during the time prescribed in the recipe. For these, you don't have to sweat it, keeping the original time is mostly OK. If you create major occlusion (e.g. two separate pans of gratin above each other), add some time by gut feeling, and check frequently for doneness toward the end. 

For sensitive recipes (roasts, custards, some cakes), baking by time doesn't work anyway. The best you can expect from a time given in the recipe is a rough guide to help with planning when to start preparation for a meal. You have to always bake these until they are done, regardless of what the recipe says. In this case, you again don't have to do any time adjustments, because you are not baking by time. 

To summarize, no, you don't really have to do any adjustments to time. Just bake everything until it is done, and if your time-based expectations were wrong, adjust based on doneness.