No, each bulb of garlic will cook at the same time. So, you can roast 1 bulb (also called a head of garlic) or 20 and the time will be the same AS LONG AS the bulbs are about the same size. The size of your garlic bulbs may affect the cooking time, though. If you have very large garlic bulbs and much smaller ones, the smaller ones will cook faster. So, keep an eye on them as they cook and, once they are golden brown and soft to touch, remove the small heads of garlic and let them cool while the larger bulbs continue to roast.