The cooking times quoted in cookbooks and recipe sheets are only accurate if you have nothing else in the oven. If you have multiple items in the oven at once, you will have to increase the cooking time. There is no rule of thumb regarding increasing cooking times for multiple items. The extra time required depends on how many things are in the oven, the density of the cooking vessels, the type of food being cooked and the efficiency of your oven.
Decide how many items you are going to cook at once. Work out if they all fit in the oven together and if the heat can still circulate around each dish. If you're cooking totally different dishes together, work out which one will take the longest, and put that in first.
Put the items in the oven when the required temperature has been reached. Check the suggested cooking time, if you are making 24 muffins instead of 6, don't change the cooking time at all. If you have doubled or tripled a cake recipe and are cooking it in a large tin, then add 15 percent onto the suggested time. If you're making two casseroles or have doubled a recipe, add 20 percent to the recommended cooking time. If you are baking a cake, do not open the oven before the suggested time, has been reached. Opening the oven early could make your cake or bread collapse, or not rise.
Open the oven after the baking time has been reached. Check if your cake is cooked by inserting a skewer into the center. If it's sticky, put the cake straight back in. If you're cooking meat, take it out of the oven and check the internal temperature with a cooking thermometer. If you don't have a thermometer, stick a knife into the center and check the color of the liquid. Chicken juices should be totally clear for it to be safe to eat. Red meat is OK to eat with pink juices, if you like it rare. For well done meat, the juices should run clear.
Put the items that require more cooking back into the oven. Estimate another 10 percent of the suggested cooking time before checking the dishes again. Use the same system as above. Generally speaking, if you're cooking two or three items all the same, in a fan-assisted oven, increasing the cooking time by 10-15 percent should be fine. But if you have more items or if the items are very dense, then an increase of up to 30 percent could be required.
Check each dish. Things being cooked on the bottom of the oven may take longer than things on the top shelf, particularly if your oven is not fan assisted. Never presume that because something on the top is done everything in the oven is. The more items you try to cook at once, the more confident you need to be, in knowing if something looks or smells cooked.