You can cook multiple chicken breasts, steaks, or pieces of salmon at one time. The amount you’ll be able to cook largely depends on the vessel you’re using. In a well-insulated, covered container—a cooler, say—your little Joule can heat up to 10 gallons (40 liters) of water. In a covered pot, it can heat about 5 gallons (20 liters). When you’re cooking without a cover, we generally recommend limiting water volume to about 2.5 gallons (10 liters).
The number of items you’re cooking will not increase cook time as long as the pieces are all separate (not necessarily in separate bags, but in bags that aren’t too crowded). Make sure the pieces of food have plenty of room to move around in the bag—if it’s jam packed, it’s too crowded. You want each piece of food to cook evenly in the surrounding water; if it’s packed tightly in the bag, food will cook unevenly. Same goes for the pot—leave enough room for water to circulate all around the food you’re cooking. If there are lots of bags jammed together, the water can’t do its job.