"Often the journey is more important than the destination." Meathead
When the whole fam damily is coming over, you might want to cook more than one pork butt or brisket, or a shoulder and a brisket and some ribs. If you have the space, it's no problem. The two questions are, how much do I need, and how does loading up the cooker impact cooking times?
I've tried to answer the question of how much do you need in my article on party planning, so let's discuss the cooking issues.
The most important concept you need to remember is that food is cooked when the center of the thickest part hits the target temp as measured by a a good digital thermometer. Any recipe that says "cook at 225°F for 5 hours" is a trap. Cook with a thermometer not a clock. Yes, you can guesstimate that a pork butt for pulled pork will take about 90 minutes per pound, but that only holds for an average 6 to 8 pound butt, all of which are pretty much the same thickness. But the exact time can vary significantly depending on the amount of fat in the meat, the breed of hog, and even the humidity inside the cooker.
The second most important concept is that the job of the warm air in an oven (remember, your covered grill or smoker is really just an oven) is to raise the temperature of the meat by warming the outside. That warmth is slowly transmitted towards the center like a bucket brigade on all sides, and how long it takes the buckets of heat to reach the center depends on how much warm air there is, what temp it is, how thick the meat is, how cold the meat is, and from which direction the heat is coming.
Let's look at an example. Let's say you want to cook three pork butts for pulled pork, each 8 pounds at 225°F, my favorite temp for butts. When you put a cold hunk of meat in a preheated oven, the air temp in the space will drop a bit. How much depends on how much air is in there. In a large smoker, the drop will be barely noticeable. In a small unit, it might be 10°F or more. Heck, just opening the door knocks you back quite a bit. So you need to get the temp back up to target. On a charcoal grill, you open the intake vent a bit. On a gasser, twist the knob. A thermostatically controlled pellet cooker will adjust itself.
Now let's open it up and add another butt. Now there are two big blocks of cold meat in there. Again, the temp will slide down a bit. But as the meat warms, your cooker can begin to climb so you need to adjust downward. If the air were perfectly still, we could measure a bubble of cool air surrounding the meat that dissipates the further away we go from the meat. But usually the air is flowing through the cooker, so the bubble is broken by convection air flow. If you have those two butts practically touching, hot air might not flow between them and they will act like one big hunk significantly changing the cooking time. But if you have a good gap, let's say at least 2", and if you adjust the oven temp to 225°F, then each hunk will cook at the same rate and take no longer than if you were cooking only one hunk.
In theory, if you have enough space between the hunks for air to flow, and if you can hold the target temp, it should take no longer to cook several clods of meat that just one. Each hunk will cook independently.
In theory. If you don't have good control over your instrument, then it is easy to go through a period of yo-yoing temps. But if you have practiced and know your instrument, and if you have a well constructed cooker with thick walls and tight doors that holds heat well, then stabilizing temp at the target is quick and easy. This process of stabilizing can take up to an hour and that can add significantly to the cooking time, perhaps half an hour. Ceramic cookers like Big Green Eggs absorb and radiate back a lot of heat and they don't leak much, so they are easy to stabilize, while cheap gas smokers have a longer recovery time.
Then there is the "heat shadow" effect. The offset barrel smoker in the picture above has a firebox on the left side just out of the picture so the heat flows from the firebox on the right and up out of the chimney on the left. The meat closest to the heat will cook faster while the rest is in the heat shadow of the meat closest to the heat. So it's a good idea to rotate the ribs in a rack once or twice during a cook so those on the ends don't overcook and those in the center don't undercook.
If you have a bullet smoker, like a Weber Smokey Mountain, there are two cooking grates, one above another. One is right above the water pan and closer to the heat source. The other is about a foot higher right in the parabolic dome. The meat on the lower rack is protected from direct heat by the water pan, and it gets cool air and moisture from the water below. The space below the meat is small, so airflow around it is inhibited. Hot air rises and goes to the exhaust vent in the dome where it pools. As a result, the food on the lower rack is cooler. On the upper rack, the meat practically floats in warm currents of convection heat, and it even gets some radiant heat reflecting from the dome.
But let's say for the sake of discussion, that you have a nice tight, thick-sided cooker with three racks and plenty of airflow around all three, something like a Backwoods Smoker. Let's say that you preheated the oven to 225°F and then you placed a butt on each shelf. Let's say that you get them in quickly and don't allow the interior to cool off too much with the door open. Then the ratio of hot air to cold meat is significant and the cold air bubble around the meat is insignificant. Cooking time for one butt will be about the same as for three butts.
Now let's say you are trying to cook those same three butts on a classic 22.5" Weber Kettle grill. You have the grill setup the way I recommend by pushing the coals off to one side of the lower charcoal grate and you have the butts crowded on the other side of the upper grate. There's not going to be a lot of hot air in there, airflow around those hunks of meat will be poor, you will need to open the lid occasionally to rotate the hunks so the one closest to the heat doesn't overcook so you will lose hot air and temp will yo-yo, and you will need to add coals at least three times during the cook. Cooking time will be several hours longer than you expect. Perhaps 30% longer.
If you are not confused yet, airflow and humidity can also impact cooking time. Try as I might, I cannot make this paragraph so that you need to read it only once. Here we go: Meat is wet and as the water evaporates from the surface it cools the meat just as sweat cools you when you are cutting the lawn. This slows the cooking until the surface dries out, a process called the stall. At higher temps and in high airflow cookers, the stall is shorter. because the airflow sweeps away the moisture and the surface dries faster. Add a lot of meat and you add humidity and slow airflow so cooking takes a bit longer. Especially if the meat is cold because the humidity can condense on the meat cooling it. To make it more confusing, if you use water pans they add humidity, and some cookers, like electrics, are very moist because there is no combustion burning oxygen so there is little airflow. Sooooooo, how does all this impact cooking time? Depends on your cooker, your cooking temp, and even the humidity in the outside air that day.
So, how does adding more meat impact cooking time? If you get it on, and the spacing is adequate, and you can keep the temp stabilized at target, if you don't block airflow, in theory it will not take much longer to cook three butts than one. But most of the time getting the temp stabilized takes time, especially if you have a poorly insulated or leaky oven, so plan on adding about 10 to 20% more cooking time. If they meat is packed in tight like in the picture above, it can take 25 to 40% longer.
Now I know you're going to ask me "how much time longer will it take me to cook a 15 pound brisket if I add two 5 pound butts?" Please don't. As you can see, there are too many variables.
The solution is simple, unless you really know what you are doing, unless you have really mastered your machine, whenever you are cooking low and slow, especially large clods of meat, start much earlier than you think necessary. You can always hold the meat at serving temp in your cooker, in your indoor oven, or in a faux cambro.
Then you will never be embarrassed telling your guests the food is not quite ready yet.